I just checked and the Vivaldi iOS app opens the official YouTube iOS app (unless you uninstall it I assume) when I tried to go to the YouTube site in it. I mostly use Safari/Firefox, is there some setting I’m missing?

Btw, I think the most useful setting for the action button is to assign it to open a shortcut folder. It opens as a popup window which at least in portrait fits up to 8 shortcuts of any kind. I have one shortcut as an “action center” which has a lot of random functions I can choose from in a menu (even triggering other shortcuts in a different folder) and the setup is relatively simple. The rest change depending on needs but mostly I have smartlight on/off triggers, app icons for frequently used apps, frequently used shortcuts, I have one that’s just the flashlight but on a lower setting than the lowest “notch” in the control center flashlight. Also one for the new Translate speech feature which you can use in shortcuts so you don’t have to assign the entire function of the action button to it. Anyway I can’t go back to just making the button toggle mute when I can have it toggle mute or do any 7+ other things of my choice but that’s just me. It feels like a weirdly customizable part in the iOS ecosystem tbh since the possibilities are pretty vast given what can be done via Shortcuts.