Basically, fundamentals can be summed up as a system for understanding how to win interactions in varies states of gameplay.
When I play Zard vs say, a bowser, I really don’t like this MU. This is because bowser has similar reach ( Ftilt goes farrrr, his jab is great, and his firebreath works like ours), but has a superior grab game (because he has a much better kill confirm and damage racker), which means I have to be extremely cautious when battling him.
I can play the MU in a few ways; slow the match down and use flamethrower to halt his movements and use Fair and my tilts to box him out, or play more risky and run around a lot to try to bait a mistake and punish with either a grab or Jab (or Usmash and Utilt to set up a juggling game).
By throwing him into a juggle situation, I can rack a lot of damage on him with my intangible wings. Bowser still has options though, and can drift towards his front and do a Bair, drift towards his back and do a Fair, Side B, or he can do a weirdly timed Dair, Down B, or go to the ledge, etc.. he’s not helpless and I have to watch his every move and react.
Bowser can respond to me slowing down the game by baiting out a flamethrower or any spacing tool (jab has little range, though that could useful as a mix up to keep bowser guessing on when I’ll throw something out), and responding with his own firebreath, or waiting til It gets weak and go for a punish. My tilts are a simple shield and grab.
Bowser can respond to my risky play by reading my attack patterns and spacing, and space himself into shield grab range, or by using his own firebreath to slow me down.
The neutral is a constant “push and pull” battle, with many different attacks needing different responses.
Also, the RPS situation I described above (could’ve went into more depth) applies to juggle situations, edge-guarding (on and offstage), on a platform, and many more.
Fundamentals is basically a conversation.
Y’all don’t have to, but these videos can help you better understand it as well. Def don’t have to watch all of them.