Title. I saw scratch.mit.edu. in another topic but that is not a mirror but a mistake so, what exactly is one? And what are they called?

A scratch mirror (I assume) is a site where it mirrors an earlier version/current version of Scratch 3.0. An example is when one Californian college/university/i have no clue I forgot who they were that were involved in the development of Scratch was able to randomly post a 3.0 mirror onto the internet. I think it’s gone now though.