It's an extension:
Each Container stores cookies separately, so you can log into the same site with different accounts and online trackers can’t easily connect the browsing.
A great idea for a feature if you ask me. For example, I have two Buffer accounts and my solution is to use different browsers entirely to stay logged into both of them. I know plenty of folks that prefer the browser version of apps like Notion, Front, and Twitter, and it's cool to have a way to log into the same site with multiple accounts if you need to — and without weird trickery.