I was wondering how the modulation in Sword Art Online - Swordland (Main Theme) from the Bbm in measure 19 to the Cm in measure 27.

Particularly I'm curious about why does the B chord in measure 23 work in connecting this transition? Also whether my understanding of Abm (measure 24) and Bb (25/26) is working as some type of backdoor chord progression to the Cm in measure 27, and how come using Abm and Bb chords while still in the key of Bbm still works during this transition?