With automatic sync on, your changes go back to Google about every 6 hours. Tick “Also bring in new contacts automatically” and new Google contacts come in too, at the level you pick (Acquaintance unless you change it). It never pushes anything you haven’t approved once.
When someone’s number or address changes, fix it once on their Nimbus card — the sync carries it to Google.
Second Google account? Sync → “+ Connect account” → Google → pick the other account. Each account gets its own row.
“↓ Get new” on the Google row shows who is new in Google before anything lands. “↑ Push” sends your changed cards to that account.