This Merriam-Webster site provides an explanation of this usage, better than one I could create (and as suggested in the comments). And you could try doing a search of am/is/are.
In case the link disappears, this is what it states:
The construction “be + to + verb” is a formal construction that is used either a) to give an instruction or command, or b) to tell what is going to happen in the future.
In the following three sentences, the “be + to + verb” construction signals that these are commands. This construction makes the sentences sound serious and formal.
You are to do your homework without watching any TV.
The students are not to travel by subway without a friend.
[Patient speaking to nurse] The doctor says that I’m to make an appointment to come back next month.
In the following sentences, the “be + to + verb” construction tells us that these sentences are about planned or scheduled events.
The plane is to take off at 9 pm.
The new kitchen cabinets are to arrive before December 20th.
Their house is to be sold right away.
Again, these sentences sound formal. If we change the verb construction to “be + going + to + verb,” they retain the same meaning but sound less formal.
The plane is going to take off at 9 am.
The new kitchen cabinets are going to arrive before December 20th.
Their house is going to be sold right away.