The main site's repo is https://github.com/patrickulrich/nostrcal and the android version is https://github.com/patrickulrich/nostrcal-android. It's still the same NIP-52 events, even for private events, they're just being wrapped via NIP-59 gift wraps.

The project originated as a calendar booking service, like Calendly, and by necessity morphed into a wider calendar project. Since it's original method was around booking I handle the gift wraps very similar to NIP-17 DM messages. I've also used kind 31926 and 31927 to address availability. 31926 let's you set your availability template for calendar booking while 31927 is to allow you to publish a "busy" time. This is primarily intended to serve as ways of blocking time against gift wrapped 31923s but can be used just to indicate busy blocks. https://github.com/patrickulrich/nips/blob/master/52.md