you can,
- kdeconnect can do clipboard & messages sync
- nextcloud (or anything else that does webdav & webcard) does calendar & contact sync
- Firefox (and chrome probably too) can sync browser history and share webpages feom one device to another
i'm not aware of anything that can remotely unlock another device, but that seems like a security risk to me (also quite hard to implement as pam is synchronous :/).
but you certainly don't need a closed ecosystem, building your own out of free components is very much possible.
Some of the stuff you mentioned seems to cover computer to computer, but isn’t relevant to computer to mobile scenarios.
no, the things I listed are how I sync my linux machines with my mobile phone.
I wasn't talking about computer to computer sync, but the methods I listed (maybe except kdeconnect, which is built for computer to phone sync) apply to workstation to laptop sync aswell.
Like it or not, being signed in everywhere with iCloud is an amazing experience. I know this because I recently tried a second user account on my Mac that wasn't signed in to my iCloud account. It was jarring.
As an Apple user I take it for granted that my watch unlocks my computer. That text copied to the clipboard on any device is immediately, invisibly available on the others. That all of my contacts and messages are just there. That my Safari history and state synchronises.
You can't 'selfhost' this stuff. You may not like Apple -- I'm not trying to convince you -- but the fact is that there are a ton of benefits that their integrated ecosystem delivers that you just can't emulate yourself.