tl;dr: Apple can see what apps (or at least what app developers' apps) are installed by a specific IP address through OCSP. It used to be unencrypted and theref

tl;dr: Apple can see what apps (or at least what app developers' apps) are installed by a specific IP address through OCSP. It used to be unencrypted and therefore visible to the internet, it is now only visible to Apple. Apple said they don't keep logs, but theoretically they could if they wanted to.