iOS doesn’t give many options to remove ads from YouTube videos. You can’t really circumvent the native YT app (at least in my experience). What’s worked for me for the past few years is:

• watching Youtube in Safari
• purchasing the Wipr and Vinegar extensions from the App store

I have not seen a single ad on iOS on YouTube this way. I’m pretty sure Vinegar is doing the heavy lifting specifically on YouTube but Wipr picks up the slack on every other website.