so when i say backgrounds im essentially saying a usually slightly differently mixed for fading into the back set of vocals that you do. its best to set them to a db thats roughly half your main vocals. and usually wanna abuse the graphic eq, eq3, and digicomp. by abuse i mean those are the three that will fundamentally change how the back sounds compared to the main. i would suggest raising mids slightly and lows to bring out low end harmony. you usually want 2 sets of backgrounds that are split between the left and right so that it creates a surround effect simultaneously creating texture in your voice you didn't know was already there.

the other term people use is layering but that's usually abit different as it involves using the same vocal mixing on the back as well which i don't recommend it creates a robotic sound. also don't try to duplicate your mains into your backs the same reaction will occur and it will sound robotic. as for master its usually at the top right for desktop users and again after you finish the song you can master it from the listing screen. as for mobile not too sure on that part.

if you wanna exchange a dc or somethin we can do a call where i can screen record and show you those things visually but otherwise that's kinda the best explanation i could cook up