Pihole is like a network wide ad/telemetry block and unbound is a recursive dns server, so rather than using cloud flare or Google to connect to a web address, it can cache or save those sites after you connect to them once with the help of one of those external parties that you visit and help to make the connection without broadcasting to other companies that you’re trying to connect to that site.

I’d say that’s it in a nutshell but I’m sure someone more technical will tell me why I’m wrong