The best solution on iOS is Yattee. You can add Piped or Invidious instances as locations and stream ad-free YouTube from there.
Another solution that doesn't involve Piped and Invidious is AdGuard. Open a YouTube link in Safari, hit actions, then hit Block YouTube Ads (by AdGuard).
If you don’t mind sideloading and refreshing the app from your PC every 7 days, I recommend (and use) uYou+ as a sideloaded app. It has PiP capabilities w/o needing premium, along with a built-in video downloader, sponsorblock, and adblock.