The playoffs will work just like they did last year, except for substituting the MaxPreps rankings for the FHSAA Power Rankings. So the 4 District champions in each Region automatically qualify. Then the next 4 highest ranked teams according to MaxPreps are awarded the 4 at-large spots for the Region. These 8 teams are then seeded #1 through #8 based on MaxPreps rankings, with 1 vs. 8, 2 vs 7, etc. The higher seed will always host in the Regional tournament, except in the first round when a District champion is facing an at-large team, the District champion will host regardless of seeding. The four State semifinalists are reseeded based on the end-of-season MaxPreps rankings, with #1 playing #4 and #2 playing #3.
So district runners-up are not automatically qualified. One District could conceivably have 4 teams in the Region playoffs (District champ + 3 at-large)
I would like to tell you how the MaxPreps rankings are calculated, but the algorithms are stored in a vault at Fort Knox, and thus not accessible to those who should actually have a clue as to how MaxPreps affects their fortunes.