It will post to all relays with the "Publish to this relay" toggle on, these are imported only once from kind:3 or kind:10002 when adding account.
If you have Autopilot enabled it will also post to inbox relays of tagged pubkeys, but only if you follow them and their kind:10002 is not garbage, and it will try to choose relays that are different from the ones without Autopilot.
There is a toggle "Show extra relays used on post preview" that will show at the bottom of the preview sheet which extra relays are going to be used: