Question FPS Animation, separate lower and upper body?

I am currently making a multiplayer FPS, and I have a question about how to animate it in such a way that the arms and head can be independent from the torso. If this was a singleplayer game, I would just model and animate the arms and hands of the player, but since its multiplayer, it needs to have a whole body.
Basically, I want to be able to have a walking animation for the legs, but do procedural animation for the arms holding a gun. The attached gif is what I would like the final result to be similar to.
I have never tried to do something like this before, so if you could recommend a tutorial, or just point me in the right direction, it’d be very helpful, thanks!


PS. On a separate note, how do you make it so that when the angle of the players look direction changes, if they aren’t walking, the legs automatically shift, like in Fortnite?