LINKING-to select a record from a field in another table

Hi everyone, I would like to ask you for help:
I have the MAIN database and then couple others (A,B,C…) in tables next to it. To fill in new records within a particular a field of MAIN, I was choosing informaton from the database A. These were linked, such that I started typing a new record in MAIN and it automatically showed me options from database A and I chose a desired option. Then I copied the whole database and this link has been lost and I do not know how to restore it. I would really appreciate your suggestions!
Thank you very much.

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