Furthermore, I'm interested in what their solution for email, calendars, and all the other Google services look like. If it's AOSP, that's a good reason not to use this ROM. I suspect it's not actually using AOSP's apps, though.
There are no built-in apps for that. The only custom apps GOS provides are Auditor (https://attestation.app/), Camera, Calculator¹, Contacts¹, Files¹, Gallery¹, Messaging¹, a hardened PDF viewer, and Vanadium (a hardened version of Chromium). Everything else is up to you.
¹) AOSP app or fork of the AOSP app.