Semi-competent Tetris player here. (I'm no TGM grand master type you'll see at GDQ, but I can definitely hold my own online.) Tetris at its core is less about knowing what to do with specific pieces all the time so much as it is being able to recognize the connective relationships they can have with one another, as well as how to react in general types of situations that will arise as time goes on in your average Tetris session. There are certainly high-level meta theories about how you should be building up your "well" of pieces, as it's commonly called, but they're somewhat contingent on how individual games are played since different versions have adopted different rules at a granular level over the years. All of this is to say, any advanced strategies you seek out should only really be done for the specific flavor of Tetris you want to play, as not all tips and strategies apply across every Tetris game ever.
So back to the two things I was talking about before. When I say it's good to recognize the "connective relationships" between pieces, I basically mean the different ways that each piece type can interact with one another to create spaces and opportunities for other pieces to slot together nicely. In practical terms, this means paying attention to your well as it grows and examing the edges for opportunities to slot in potential pieces as they appear. This also means that you should memorize the shapes each piece adopts as they're being rotated so that after a while, you immediately know what sort of options are available to you positionally as soon as a piece appears in your queue without having to manually rotate it and reassess the board. Most pieces only have two shapes at most that they can adopt while being rotated, with the J and L pieces being the big exceptions and taking a little more time to get down pat. Eventually, doing all of this will allow you to start planning ahead for how you want to fill out your well before you even take control of a given piece in the queue.
Learning these rotational shapes as you keep playing more and more rounds will also help you recognize different situations where you can safely clear, say, two lines from the board or three lines, something you need to eventually be comfortable with doing when needed so that you don't end up screwing yourself over while waiting for an I piece to appear so you can get tetrises. At a more advanced level, learning these rotations will also help pieces navigate black space between pieces that you'd normally be blocked from entering, but that's not something you should concern yourself with learning in the very beginning. And more broadly speaking, learning these rotations and the possible relationships that each piece can have with each other will give you the insight to get out of troublesome situations quickly and reliably, such as when you have too big of a piece buildup in part of the screen.
In terms of when to hold a piece, my general rule of thumb is to hold a piece when possible if it either won't make a useful contribution to my current well (ie: won't cause any major gaps or start a pileup of pieces). There are also times when I hold a piece when I'm specifically trying to set up a particular shape on the board or when I'm trying to clear out a few lines in a specific way, but generally I use it as a defensive move myself. Sometimes this'll back me into a corner, especially when the reserved piece is the same as the one I'm trying to swap out, but the system is designed to cause that to happen, so at that point, you've just got make the best of what you have.
Really, in a nutshell, Tetris isn't about playing a perfect game so much as it is one that tests how you'll react to mistakes and inevitable gaps in the board that arise along the way. A lot of the skills I've described are things you'll naturally pick up over time as you play the game. In my opinion, most anyone can become a decent Tetris player with enough practice, as there are only a handful of core skills you need to build up and the game is designed in such a way that you'll intuitively catch onto them with experience. For what it's also worth, I think a lot of "modern" Tetris games, while very well designed, can be bad about teaching you the absolute core fundamentals of the game because of the additional gameplay flourishes they've introduced over the years and I suspect that Tetris Effect may be the same to an extent because of its own unique wrinkles that it introduces to the formula. So, what I tend to recommend to people who want to just focus on honing their basic Tetris skills without needing to worry about stuff like the hold system is to play one of the more basic older installments like the Game Boy one or one of the simpler variants that you can play for free on Tetris Friends. Then, as you get more and more comfortable with how the pieces interact with each other fundamentally, you can slowly start to play more advanced versions and slow incorporate those additional features into your overall strategy. Like I said above, each version of Tetris plays at least a little bit differently, so there can be a bit of an adjustment period as you switch between games when it comes to, say, how fast pieces drop, how quickly that speed increases, and how much time you have to rotate a piece once it's on top of the board (if at all), but playing a simpler version of the game will overall give you skills that can be transferred between versions.
Long post after a really long week at work, so hopefully that makes at least some sense. Good luck! It's a beautiful game to play in most any incarnation! c: