I know nothing about what any of this does, or how 2 use it. Its looks amazing I just have no idea what the hell I'm actually doing.
what I want to do is convert a .schem file into .schematic file so that i can use it in my 1.12.2 world, but idk how to do this. and got some advice?
Then extract the zip, open "Minecraft Tools.exe" , and on the tool selector window click on the one called "Minecraft 1.13+ (or very old) to 1.12.2- world converter" , then on the actual world conversor, just select your desired world and then press the "Convert" button (the one with a command block), then just wait until it pops a message box telling you it has finished. Then just start Minecraft and look at the bottom of your worlds list, it should be there if all has gone well. And by the way, none of my tools will ever modify any of your original files since all load them always as read-only.
I really hope that this might help you a bit, even if it's a lot late (sorry but I didn't had internet available for a few months).