Business portfolio, ad account, campaigns: ownership versus spend
"The business layer is about ownership and access; the ads layer is about campaigns and spend."
All tools > Business settings shows one card per business portfolio, each with its own ad accounts, pages and people. Underneath, the ad account (chosen from the selector chip) is the container for campaigns, drafts, reporting and billing. The gear icon opens ad settings (account id, linked pages, notifications); Billing and payments holds the tax prompt, available funds, daily or weekly spend and the payment method.
Knowing which layer owns what tells you where to go when access or billing breaks.