Basically you should have learned fundamentals before applying random tech skill because learning wavedash and shffl is useless if you dont know how to use it. Everything basically comes from predicting your opponent's movement. Depending on the space you have between you and the opponent theres things you can bait out of him. Either jumps, dashes, rolls, wavedash, shielding, sidestep.. all of those you need to pay attention constantly and train those situations until you can punish without problems. Its never about reactions, it's always a prediction which makes it seem like in a video "that guy has really good reactions" when in reality its not that.
Rules of fundamentals is basically this:
If he's above you on a platform, you can watch out for falling aerials, shield drops or being baited on a platform.
If he's on the side then you try to read the options i mentioned above in the paragraph.
If he's below you then you can bait his double jump and downair him for example while staying on the platform or try your best to come back into neutral because being above someone is generally really bad.
But then when you punish moves you really need to know your own moves and his moves as well, thats how you train matchups. Theres multiple responses to beat out 1 move and yet initially those moves begin by dashing or jumping.