how to have select list as a field draw from a content type?

I have a content type buildings and one called rooms. There are multiple rooms per building. I have a form for room and I want to have a list the user can choose from that is a list of the buildings, that is generated from the list of buildings.

How?

The only type of list I see is tags and I want this automated, not manually add to the list or tags.

Thanks in advance

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