I keep things very simple. I have a Single Source of Truth for all my contacts. Right now, I'm using a contact app on my phone for this, but over the years I've used different things. The exact method isn't important to me, what's important is having an authoritative contact list.

I don't sync anything with anything. I look up a contact and enter whatever detail I need manually wherever it's needed. I lean a lot on the frequently-used autocomplete lots of applications have, too, but that's a convenience that I don't take as authoritative.