I want to make static position for the trash slot, but I can't, right? I want to place it in one spot in my inventory, without any position editing later on

I found the file, but why is it not in the config folder?

Because it is a frequently changing runtime-options file rather than a static configuration file for the mod. It updates whenever the player moves the slot, enables/disables it in a screen, or locks/unlocks it. It is also not meant to be shipped in modpacks, while files in the config folder generally would be.