Don't mix up hitting your opponent's shield or actually hitting your opponent (grab or shield poke) with shield pressure. The goal of shield pressure is to land a hit - or in other words, turn their shielding into a punish, but then it is a punish and no longer shield pressure. It is the gap between your opponent shielding and your opponent giving you an opening that you can punish.
First, you must understand that shielding greatly limits your options. You lose dash and all your ground attacks that aren't up-smash, grab, and maybe your up-b. Even then, those moves can't have the slight spacing adjustment that dashing first allows. Same goes for your aerials. Not only this, but even the option to wait is taken away (or at least it is limited). Your shield will break if you do nothing.
That said, shielding players can still punish you if you do something stupid on their shield. They can also get away quite safely a lot of the time by wavedashing away (this is where stage positioning is huge - wavedash away starts to suck when it puts you off stage every time). This is why you have to do safe things on shield - like spacing from far away, doing crossups, and doing frame traps / safe-on-shield attacks if you want to hit their shield. As fox, you have tons of capability to do all these things.
Now it is to determine the goals:
- Keeping your opponent in shield is still winning the situation (their shield will break, so their hand is eventually forced).
- Managing your opponent's retreat paths allows you to maintain pressure even if they escape for now. Particularly, blocking the roll / wavedash in is a good way to force them to retreat further and further from center stage. It is actually quite similar to how shield pressuring works - you leave them only options that are equal to their current situation or worse. Just remember that them retreating away from center can be considered a small victory and allowing them to move into center is basically throwing away your entire advantage.
- Doing ambiguous and safe hits on their shield keeps them guessing. You can't just start doing something when you don't know exactly when the next hitbox will be in your face.
- Actually hitting your opponent is the end state - it is where your shield pressure ends (they aren't shielding any more). While it is the ultimate goal of this whole process, it is not actually part of it (at least not the exact act of shield pressuring - punishing an opening from shield pressure is different from pressuring your shielding opponent).
