If you run more than one account on a site, a personal one and a business one say, Proxy keeps a separate session for each. Every account has its own tabs and its own login, so an agent working as one never has to sign the other out, and two accounts can be busy at the same time without waiting on each other.
Logins come from your own Chrome, through the Proxy extension for Chrome, and only for the sites you list in Settings → General → Proxy (X and Reddit to begin with). Google account logins are never copied. The first time an agent uses an account, Proxy copies that site's login over and, on X, checks it's signed in as the right handle, switching if it isn't.
The Proxy panel lists each account's tabs under its name, with who it was last confirmed signed in as. "New tab" lets you sign in by hand, and "Re-import login" copies Chrome's again. If a site needs a login Proxy can't copy, you get a notification asking you to sign in to it in Chrome.