I know that the command / testfor is pretty important, especially with maps but I don't know anything about it…
Can anyone please explain what it is and how to use it?
Not at all. This command does not exist. Deal with
execute if
But it exists in 1.12.2
Yes, but why should you play on 1.12?
Because I find the version better?
The 1.16 has - drum roll - more features.
I know but I still find it better