The geo sync updates are handled with distributed keys over internal api, here is the documentation for powerdns around it: https://doc.powerdns.com/authoritative/dnsupdate.html#dnsupd... so the updates are pushed and updated to primaries if the update is done over DNS and if done via API there is a normal replication function.
right now there is no anycast available, possible in the future
Thanks for the answer. I use DB replicas (so all dns servers have a common backend), but only one accepts writes.
I still can not figure out any economical way to roll out anycast.