"go over the edge" and transform into some for of demon, there is no demon seed eating/demon merging but I think the orochu whatever demon may in some way trigger the transformation "in the new version". The old version leaves a lot of details out for whats happening, where as the new one is giving more background and building up villains unlike the old version
When Garou transforms into the demon, the fight is like... several chapters long if I recall. Much much stronger than Lord Boros, If I recall... garou transforms several times during the fight, and takes many "super serious" huts and keeps fighting
rechecking, it looks like his "hate" transforms him, no demon seed etc
Transformation starting dialogue
Chapter 76 - Page 10
reappears as Demon Garou
Chapter 77, page 13 start
Saitama vs Demon Garou fight start
Chapter 86, Page 15
Saitama goes serious mode
Chapter 90, page 2
Fight somewhat ends at Chapter 92, page 19
Garou ark ends at chapter 94 and 95 starts a new ark with a new city (since the garou fight pretty much obliterates the entire city(s), hero association etc.