it's chapter three of the first volume.
That's the protection squad one though, I don't think they're related
Ah sorry i made a mistake. Its the sequel to the third chapter of the secind volume. It's the only chapter in the volumes that has a sequel lol
The chapter 5 of the second volume doesnt havd a second part. The "second part" In the title refers to the game the protagonists reincarnates to. In the game in the first part you defeat the demon king bla blah blah and then you move on to the second part of the game, where the best friend is the new last boss/demon king. A lot of games like that habe different 'stages/levels/chapters/parts'. If you read the chapter again closely they explain it. It can be a bit confusing if you dont know the concept
TLDR; Page 11 and 17 mention it and the titles says its the the second part of the GAME. There is no part 1 or 2 in the manga
Where's the first part???