Seems like others offered good resources, so I'll address this thread somewhat more directly
Push-fold charts are different because different people have different play-styles and strategies. Although only my opinion, I believe that learning some charts and ranges is useful for getting the gist of what is standard, but shouldn't be followed too rigidly in most cases because I'll adapt slightly based on the situation. Charts like push-fold or opening ranges are guidelines in the sense that they might be mathematically correct given the parameters set, but they might not be identical to the actual situation you get in your game.
Even when you should be push-folding is up for discussion. 15bbs is actually a decent amount of chips; you could play nitty and fold a ton of orbits before you get blinded out (seldom best strategy, but just showing you can survive off of 15bbs long if needed).
You can even be playing non-shove-fold profitably lower than 15bbs, but this takes more skill to finesse because you want to avoid situations where you get pot-committed, or the opponent can
bluff you with any two cards profitably. In this more advanced non-shove-fold under 15bbs, it would involve lowering everything from your open-raise sizing to your continuation bet sizing. Shove-fold is the more simple approach which makes it less likely someone can exploit you.
Shove-fold range also depends on if there is an ante or not. With an ante in play, we have better pot
odds (incentivizing shoving to win blinds and antes), so shove-fold poker with 15bbs or less here makes a lot of sense. Without an ante in play, shove-fold with 10bbs or less is more standard. Again, these aren't "rules" though - people will adjust and also use different playstyles/strategies.
In the free cardschat 30 Day course, they touch on shove-fold in a lesson or two and at least another day or two is about ICM implications which is also a more advanced variable to consider as well. The course is worth checking out even if just those lessons, but why not stay for the others too?
https://www.cardschat.com/forum/learning-poker-57/cardschat-training-course-455641/
