what's the difference between all in one, better repack rx7 and koikatsu party ?
Better Repack is the japanese version of the game, with a lot of mods and additions most people like. Also already comes with the DLCs. Some of it is machine-translated though, which is not always the best quality.
All In One is... Better Repack, but downloaded all at once.
You also have the option of downloading it split in many files in case you need to. You need to have them all to open them, but when you get them all it will be the same thing as the all In One version.
Party is the official translation, the one you can actually buy on Steam. It has no mods by default, and the only DLC officially released is the free one to add the Sorty Mode and other things Steam does not allow in the official game.
Two of the japanese DLCs you can just put in it and some mods will Machine Translate it, but the third one cannot just be added.
There is a Better Repack Party that adds all three DLCs plus some mods. But the original creator of the BRs said it would not be updated, so either move to the japanese version or use ManlyMarco's HF Pack, which does not have DLCs but is constantly updated for both the original and Party and, as I tested myself, seems to be able to use together with no problem.