I agree with @Great_Moustache, I’ve run a superhero campaign from levels 3-10 and the simplest way to handle it is that if the character is powerful enough they just succeed at any task which wouldn’t cause them to struggle. “Every roll matters” after all. This especially matters outside of combat. If your version of the Flash can circle the world in a matter of seconds, then just let them; explain to the player that the rules in combat are an abstraction and describe how blisteringly quick they move (even if it’s only 30’ more than their allies) and they shouldn’t have any issues.
When you have two characters of vastly different power in opposition, you can just say that without any outside factors to level the playing field the more powerful character wins. They don’t necessarily win overall, but if you have Daredevil trying to take on Hulk in a fist fight it’s pretty obvious which one is going to come out on top so there’s no point in rolling for it regardless of what their stats look like (on the other hand, if Daredevil is trying to sneak past or trick Hulk then that might be a good time to break out the dice).
If you have two different but not incomparable power levels clashing (e.g. Captain America versus Hawkeye) you can look to the Tech Levels for inspiration and give advantage and disadvantage appropriately when the difference in power is relevant. That’s about as complex as I’d go in terms of bringing mechanics into it.
That pretty much covers how I’d do it. I’ll just wrap up with a warning that these kinds of characters are often very hard to challenge while still remaining fair to them. There’s a reason Superman stories need Kryptonite, and why most powerful villains are either brutes than can take on a whole team at once (and usually overwhelm any defensive powers of the heroes) or masterminds who the heroes never meet until the end; if that wasn’t the case the heroes would likely deal with them as soon as they met, and as the GM you can’t just write into the narrative that the villain escapes, because the heroes have agency and want to be able to act to prevent it. No fair telling them their ideas won’t work just because you’d planned for this to be a recurring villain. This isn’t a reason you shouldn’t run this game, just something to be careful of.