A role says what someone may do: text, design, products, forms or just viewing. That limit also applies to that person's AI. If Priya may only change text, her AI can't change the design, not even inside a page.
A company is the place where sites, domains and invoices belong together. On the Site plan at most three people work on a site, all as administrators. On Pro and Shop you work with as many people as you like, and with roles.
Web designers build in their own partner account and hand the site over to the client. The client becomes the owner or the payer, and the designer stays on as maintenance partner if that's what you agreed. Until the client pays, the partner pays, or asks us for a code that covers the build.