I'm not sure you can on iOS? Or not in a straightforward way. If you can block YT ads on ios, would love to know
Using YouTube in Safari with AdGuard works. Not sure if it’s possible to block ads in the YouTube app.
A combo of StopTheMadness and a pi-hole works well for me.
Safari extensions "Vinegar" and "Baking Soda" are also great at turning pretty much any web video player into a native html5 player and automatically blocking autoplay.