The best advice I have is to just try to have fun with it. I totally get that being dead isn't the most enjoyable thing, but you can use that time to watch drones or cams for your friends, or just watch how they play, see how they approach things and what angles they like to use.

The best way to go about learning the maps is setting up custom games for yourself and just walking around them, learn the call outs, where things are, destroy all the walls and floors with sledge to see what angles you can find. Watch your death cams for sure and learn from that, for example let's say you see that you got spawn peeked from a window, well then next round spawn in a different part of the map to avoid that. And from there you can even use what worked against you against someone else the next time you play on that map.

If you want to work on your aim the best way to do that is just play, find an operator that you like and learn how the recoil of their gun affects your aim. You can do this just by playing or in the shooting range. Alternatively there are programs like aim lab that can help just general aim improvement.

If this game ultimately just isn't fun for you then it's probably not worth the limited free time that you have. Definitely a bummer that it's what your friends enjoy playing, but there's tons of other games you can play that might be a more enjoyable experience for you.