I'm trying to load a few Javascript files on every page of my brightspace backend. Is there any place where I can load/inject these fiels?
Unfortunately the LE is very limited in where you can inject scripts. An approach we commonly take in our organization is to use any description field to insert an iframe. That iframe only has the script in it and because it is same-origin can access the top window. Otherwise if there is no description field we use widgets for homepages. You're still left with tools and certain areas of the LE that do not get the scripts though... I hope that helps!
In summary, where we've done it: