The account behind sync
A calendar provider is an account in your calendar system that you connect to Naboro: a Google account, a Microsoft 365 account, or CalDAV credentials. Naboro doesn't talk to Google or Microsoft on its own. It acts through this account, and it can see and change exactly what the account can see and change, nothing more.
Providers belong to a company. Each company in a building connects its own, so no company ever touches another company's calendars. A company can also connect several providers, for example when some room calendars live in one account and some in another.
What Naboro uses it for
Naboro reads meetings from the connected calendar and writes bookings back into it, always within the account's own permissions. A meeting created in the company's calendar becomes a booking on the room, and a booking made in Naboro shows up in the calendar, so both sides stay in sync.
A room missing from the calendar list is an access issue. The provider account simply doesn't see the room's calendar yet. Follow the guide for Microsoft 365 or Google Calendar to add it.