Once you have the .dmg installer:
Why would a Mac admin choose v16 over the modern Claris Pro? These features were peak performance for the era. Filemaker Pro 16 Download Mac
If you have the software installed on another computer but lost the key, you can often retrieve it using the "About FileMaker" menu. Once you have the
Cause: You are trying to open a .fmp12 file saved by FileMaker 19+. Fix: The database structure is too new. You must downgrade the file using a middle version (open it in FileMaker 18 first, then save as clone). License Entry: