However, neither Adguard nor DNS based blocking will block ads in the iOS Youtube App.
Vanced is a modded YT app, while NewPipe is open source and has a background player too, with download feature.
I recommend going with the APK version of NewPipe as the FDroid install often takes a while to catch up to breaking changes on youtube.
https://archive.newpipe.net/fdroid/repo?fingerprint=E2402C78...
If, for some reason, I really wanted to download a video to have a copy of it forever, I'd use youtube-dl
For example NewPipe (Android) and FreeTube (Windows MacOS, Linux)
As someone pointed out recently, [1] this presumably breaks the YouTube terms of service, as presumably does any use of youtube-dl.