Many out-of-key chords are chords "borrowed" from the parallel minor key. Each major key has a parallel minor key (the minor key with the same root note). For example, the parallel key of C major is C minor.

You can turn major chords into minor chords by swapping a major chord for the minor version of that chord. In a major key, the minor four (iv) and minor five (v) are borrowed chords, so when you play an F minor chord (Fm) and G minor chord (Gm) in the key of C major, you are borrowing these chords from the parallel minor key. Any chord can be borrowed from the parallel key. The "flat seven chord" (bVII) is another common borrowed chord from the parallel key. The flat seven is an interesting chord, especially when you add the seventh and it becomes a dominant 7th that is ready to go home to the one chord.

The song is in the key of Eb major. Chords from the parallel key are used to differentiate between the pre-chorus and verse. For example, the verse starts on the one chord (Eb) while the pre-chorus uses the minor version (Ebm). The chorus has the minor five (Bbm) and flat seven (Db), and makes use of other out-of-key chords, such as the secondary dominant and chromatic mediant.

https://youtu.be/cbK9cVByhXQ