Enough to know how apps like "Online Accounts" work and which apps activate and sync contacts, calendar events, tasks, etc.. So you can install them and get the phone syncing everything with your Nextcloud and laptop.

In Mobian Phosh you install Evolution, which is a desktop app. Sign into your email and Nextcloud using Online Accounts, and that will get contacts, calendar and task notifications workingvand syncing. So basically how you would do it on a laptop.

Then you use Linux mobile apps like Geary for email and Contacts. Nextcloud desktop client renders good on  phones, so you can use  that.

Linux phone run desktop apps, so you can do many things, but the interface doesn't usually render well. Telegram needs tweakinga little , Signal does good. VLC will Chromecast for you, but the interface is near impossible.

Linux phone can do a lot because of desktop apps, but it's painful.