I use CalDav for calendar, contacts and tasks sync and share with the rest of the family.
Server is Nextcloud with WebDav/WebCal/CardDav and DavX5, Etar calendar, native contacts and OpenTasks on android. On Linux WebDav to access and sync Nextcloud files.
How do you use them?
Edit: Added CardDav for contacts.
I use Nextcloud with DavX5.
I tried getting my MonicaHQ install to sync but DavX was giving me a hard time connecting to both at the same time.