I'm creating a list with the keys of the songs we play in the band, and I found a song, where I'm not sure what it is. Note that this is just an example.

In general, I'm finding for a way how to get the key of a song.

Example

The song is Amsterdam from Nothing But Thieves and the chords can be found here.

Just to be complete, here are the chords (without repetitions):

Verse:  Bb Dm Gm Dm
Chorus: Eb Gm Bb Cm

Is the key one of the major chords of the chords above and can be kind of randomly selected by the creator of the song, or is there some rule or guideline to know the key?