The current onboarding process within the game for new and/or free to play players is very bad. My brothers tried the game for the first time last year when it first went free to play, and seeing the way the game handles new players was baffling. If I wasn't there to answer their questions and point them towards certain things, I can't imagine they would have stuck with it for any period of time.
The first of your questions I want to address is DLC. Forsaken and Shadowkeep are currently the only two paid expansions. Destiny has one big expansion each autumn, and varying levels of content releases throughout the year. There were two smaller expansions in the first year, but both are now included in the free to play experience (though essentially all of that content will be leaving the game in November). Year 2 used an annual pass which included 3 seasons of content released throughout that year. That content is now part of Forsaken, though as with the year 1 content most of it will be leaving the game in November.
The year 3 and still current method for releasing and paying for seasonal content is the season pass model. This serves two functions, the first being similar to a battle pass with 100 levels of stuff to unlock, the second function being granting access to seasonal activities. However, seasons within year 3 did not have content or activities persist beyond that season. While activities released during mid year seasons in year 1 and year 2 remained in the game, activities associated with seasonal content in the past year went away as the season ended. As such, the only content you could buy currently that isn't included with gamepass is the current season pass for $10. I wouldn't recommend this, as this doesn't encompass a ton of content (most of the seasonal content is freely accessible) and the season ends soon.
The next big expansion, Beyond Light, will release in November and will be included with gamepass, though you will need to buy the season pass for $10 in order to access some seasonal content not directly associated with the expansion as well as the premium tier battle pass rewards.
As for how to get started with the game, the main thing I would recommend is to seek out the campaigns. I say seek out because the game will not direct you to these and their existence is not surfaced to you in an obvious way anymore. While some are better than others, they are all worth playing in my opinion. There are 5 campaigns currently in the game the game, but 3 of these will be removed in November. The campaigns are:
The Red War: This is the vanilla campaign that launched with the game. This is probably the longest, with high production value. It might be a little weird currently because many skyboxes in the game have changed to reflect the current state of the world and the imminent loss of 4 planets, but just ignore the big black pyramids floating over some planets.
Curse of Osiris: Regarded by the community as a low point for the series, Curse of Osiris is the first of two "mini campaigns." While the associated expansion is not well liked, much of this was tied to the poor state of the endgame and overall state of the game at the time. The campaign itself is fine.
Warmind: The second of the two mini campaigns, Warmind is where the game started to turn a corner and improve. However, like Curse of Osiris this is largely a result the overall state of the game, with improvements made to progression, additions of longer term chases like exotic catalysts, and changes to the weapon and ability sandbox, which aren't really relevant to the campaign when experiencing it 2 and half years later. The campaign itself is also fine.
Those first three campaigns will be removed from the game when Beyond Light launches. In order to play these campaigns, you need to go the NPC named Amanda Holiday in the hangar area of the Tower and pick up the quests from her. Taking place after these campaigns are the final two.
Forsaken: This was the big the turnaround, Destiny 2's Taken King. While much of Forsaken's strength lied in changes to the core game, such as the re-introduction of special weapons and addition of new supers, the campaign itself was also fantastic. The tone was noticeably less silly and overall the story finally felt like it was going somewhere and building upon the deep and interesting lore within the actual game. The Forsaken campaign is the best of them all in mine and many others' opinions.
Shadowkeep: This was the most recent campaign, released last year. In terms of length of production value, it sits somewhere in between the mini campaigns and the two big campaigns. It's pretty good, but not at the level of Forsaken. It does, however, set up some things that definitely feel like they are going to be explored in the forthcoming storylines.
After playing the first three campaigns, I would recommend engaging with some of the quest the game will give right after creating your character. Many of these will be quests from the first year of the game that reward you with exotic weapons just generally get exploring the planets and seeing the sights. After all the campaigns is when you probably want to start getting into some of the endgame stuff.
You'll naturally be increasing in level up to the soft cap (Power Level 1000) as you play, and that's when you'll want to start doing powerful reward activities. These will show up on the map and once you hit the soft cap and will be sources of higher level gear from that point forward. At the base power level you start at, you'll be able to do essentially everything in the game from year 1 and year 2. As you get closer to soft cap, much of the year 3 content will be playable. It is only for high level nightfall strikes and the newest dungeon that you will need power levels beyond the soft cap.
Once you get into the endgame, you're probably going to want to find some people to play with.
Lincoln Force is the primary GB Destiny clan on Xbox: https://www.bungie.net/en/ClanV2?groupid=1723693
You can also find folks in the GB Destiny discord server: https://discord.gg/VZJhHrx
The core 3 activities in the PvE endgame are:
Nightfall Strikes: Each week a specific strike will be featured as the Nightfall: The Ordeal strike. This strike has various difficulty settings, each adding more modifiers and increasing the enemy power level. All level of the nightfall have champions, elite enemies that have unique abilities that need to be countered with specific weapon mods that are acquired from the seasonal artifact. The two lower levels of the nightfall have matchmaking, while the higher levels will require you bring 2 teammates.
Dungeons: This activity type was added in Forsaken, and are essentially scaled back raids for teams of 3 rather than 6, with slightly simpler mechanics. There are 3 dungeons in the game currently, and they are all fantastic. You're going to need to form a team of 3, as there is no matchmaking. Two or the three reward powerful drops, while two of the three (not the same two) have quests for exotic weapons associated with them.
Raids: Raids are the core endgame experience, and the IMO the best content the game has to offer. There are currently 7 raids, and they are all worth doing. Raids can vary in size and scope quite a lot, with some of the smaller ones being quite short, and the longest being a marathon with 4 bosses spanning 6 total encounters. All but the newest raid (Garden of Salvation) can be done at the base power level, and even Garden of Salavation is set below the current soft cap. You will not have to grind out any powerful rewards to do any of the raids. You will however, need to find a group. I don't play on Xbox myself, but taking a peek at Lincoln Force, it appears very active. I counted like 50 people active in just the last 10 days, so you should be able to find like minded folks to raid with if you choose. I wholeheartedly recommend raiding, it is unlike anything else in the FPS sphere, and the best thing Destiny has going for it. It is also worth noting that 5 of these raids will be removed in November (Last Wish and Garden of Salivation are staying), so if you want to try them out you have somewhat a ticking clock.
Aside from the endgame stuff, there are a bunch of exotic quests, secret missions, and of course PvP and Gambit (a team vs. team horde mode style mode with a limited PvP component). There are also vestiges of old seasonal content such as Escalation Protocol and Blind Well (public horde mode activities), Reckoning and Menagerie (matchmade raid-lite style activities) and forges (matchmade horde mode activity).
Overall, there's a ton of content in the game (though a lot will be gone soon) and it's just fun to shoot enemies. I recommend trying out the campaigns, dipping your toes into some of the quests, world activities like adventures, and strike playlists to see if taking on some endgame activities is something you might be interested in. Dungeons and Raids are, in my opinion, the best cooperative FPS experiences you can have in any game. If you're enjoying the shooting and overall gameplay, trying a raid or a dungeon is definitely going to be worth your time.