I’m still not clear about how exactly this new feature is decoupled from sync. Where is the source of truth for the sync setting — is it in cloud or locally in

I’m still not clear about how exactly this new feature is decoupled from sync. Where is the source of truth for the sync setting — is it in cloud or locally in every browser profile? If I have chrome sync enabled in browser on desktop a, then login to gmail in chrome on desktop b — is the browser history on desktop b now synced ...? Also curious whether sync implies that old browser history in desktop b is synced to my account or is it guaranteed to be the case that “only browser data collected while logged in” is eligible for being synced between devices ...?