My question is basically what the title says: does my ISP know what sites I have visited if I am using Tor?
I know that my ISP can know that I am using Tor. In addition I know that my ISP cannot read the data leaving or coming at my PC. Can it however know what sites I have visited, e.g. can it see that I am searching something on Google?
If you're using Tor and it's all working correctly then your ISP can't see what websites you're visiting.
Tor hides the identity of who you're connecting to (the webserver), so no issues there. Additionally, since Tor encrypts your traffic your ISP can't see your HTTP requests, so they can't see what websites you're trying to download.
There's also the issue of stuff like DNS. If you try to resolve a URL using DNS in the clear (without using Tor) then that reveals what websites you intend to visit.
Actually you need to run Tor as a router, only then you'll never leak a DNS info, but there's also a trick with persistent cookies: if you've visited some sites from your PC before going through Tor or logged into your impersonated profile - it can be used in metadata processing to deanonymize you even if you're behind a Tor router.
