GA4 guru Dana DiTomaso shared some interesting insights about Atlas:

• It's built on Chromium, so it shows up as Chrome in the user agent string
• Even if bookmarks & browsing history are imported from Chrome to Atlas, cookies don't come with - so visitors using Atlas will show up as New Users in GA4, even if they're not
• You might notice anomalies in New User/Returning User/User Lifetime metrics in GA4, especially if your audience includes a lot of Mac users and early adopters

The way new vs returning users is determined in GA4 is super squishy anyway, but it's still helpful to be aware of in case you see notable changes in acquisition reports starting around this time.