You're right, it isn't strategy; it's actually a tactical game. This isn't a joke or pun about the name, there is a difference between strategy and tactics, and the game is thus aptly named.
Strategy is long term, a plan laid out.
Tactics are done on a more immediate level, devised and executed according to the situation at hand.
In TFT, you don't start with a specific build in mind, because you may not get what you need for that particular strat and trying to force it can cause you to lose very hard more often than succeed.
A huge beginner's mistake when jumping into TFT is thinking that you just shoot for 1st place, and if you can't get that you lost, and you either suck or just got screwed by RNG (or alternatively, those who get first were just lucky). Due to the significant amount of RNG in the game, you aren't just relying on luck to hit 1st, you are trying to get in the top 4 spots more often than not over many games (or whatever place nets you +LP).
You're definitely subject to good and bad luck, but you're able to mitigate the RNG, so that's what you're trying to do the entire game. You maximize your chances to get what you need through tactics such as buying up all Cost X champs before rerolling if you're looking for a specific Cost X, and deciding when leveling up is better for immediate power on the field or is more efficient for rolling for higher cost champs.
There are a lot of subtle things about the game to improve your odds of winning despite the RNG, you just have to do some research and learn from skilled players, and practice yourself.
