It's a group sign in, not a separate sign in. All accounts will share the same cookies... That's not what I need. Moreover, there is weird requirement:
If you sign out of any Google product while signed in to any account, you will be signed out of all your Google Accounts at once.
I need a completely separate sign in into several Google accounts at the same time. No sharing cookies, no sharing any data... Google is dancing around the problem, not solving it at all. For using multiple accounts at the same time they suggest several workarounds, that include: using different types of browsers, using "incognito" windows (they all share data between them, which is wrong IMHO) and using different user profiles.
BTW, clicking on the link offered by Google on their page ("multiple browser profiles.") crashes its own Chromium browser with this message:
Keep it simple, it'll become complex by itself...