I assume this is hyperbole (I'm not sure what magic you're expecting...maybe something like intents where it asks you every time which account to use?), but the logic isn't complicated. It just uses the first account you logged in with. I'm usually signed in to two accounts (and have a gmail tab for each open right now), so I just sign in to the one I do my primary calendaring, etc in first.

If you really want to keep them separated, you should just use Chrome's multi user accounts or their equivalent in other browsers.