Installed tonight on my pi4. When using raspberry pi os I had no ads in YouTube. I put my pi in my shed with the 4g router.
Router is set to give dhcp with the dns of the pi4 and this happens.. I had a load of vids with no ads. Websites with no ads. Now I’m getting ads again in YouTube but websites are ok. Raspberry pi os is setup as static. The web admin works fine.
Pihole blocks ads by blocking domains. Youtube serves ads from the same domain as they server the video content. Pihole can't block these types of ads. You need a browser plugin (like uBlock) to filter on URL level.
Many have been trying with very elaborate methods to block youtube ads - but didn't success (for long).
Search the forum for others that failed. One example.