A DNS based blocker like pi-hole, nextdns, etc, can't block YouTube ads. Or at least it's very hard to it without breaking something as YouTube often uses the same subdomain(s) to load ads and videos.

I use Adblock Plus on iPhone - can be downloaded in App Store and works fantastically with mobile Safari. I even replaced the Youtube app with a home screen bookmark and I haven’t seen an ad since

Not with the YouTube app, but with the browser, you somewhat can.

I used adguard and it works for like 80 to 90% of the time. For the time where ad did show up, I just reload the page and it is gone.

Brave browser blocks most, if not all YouTube ads, one of the main reasons it's my default browser on iOS

DNS-based adblock is mostly, but not totally, effective.