One person, one email, one password. The companies hang off the login, so an accountant with eleven clients and an owner with three businesses use the same thing.
Choosing one
Signing in shows Choose a company when you belong to more than one: every company you have access to, your role in each, and a search box once the list gets long. Pick one and you are in it.
If you belong to exactly one company the screen is skipped — you land on the dashboard as before.
Moving between them
The company name at the top left of every screen is a button. It opens the list, and See all companies takes you back to the full picker with search and status.
What you can do inside each one is your role in that company, not a single setting across all of them. You can be the owner of your own business and an accountant in a client's, and each company decides what you see.
Getting into somebody else's company
You do not add yourself — the owner invites you. Settings → Users & permissions → Invite user, with the role Accountant. Once you accept, that company appears in your list next to your own.
What each company is billed
It depends on the plan. Core and Complete cover one company. Multi covers up to ten on a single subscription — one card, one bill, one login.
For an accountant this rarely matters: each client pays for their own books, and you are only a member. You see the billing state of the companies where you are the owner or an admin, and nothing in the ones where you are not.
If you run several businesses yourself, the article Run several companies on one subscription covers how one plan pays for all of them.