Feel like a Noob, need help

[RavenStorm]

So I have played many of the COD games, use to be on the Wii and played in an awesome group, and though I was not the best, I was good. Anyhow, been away for awhile and came back to the COD Modern Warfare and I have been playing for a while but I am so confused, on Mobile I can play BR and 8 out of 10 games I come in top 5 or better. On the computer I am lucky to be survivor 75, I can hear someone coming, crouch, and as soon as they open the door I can spray them yet I am the one that dies. I am sure the guns have something to do with it but good grief.

so here is my question for help, the custom loadout, how do I unlock the guns? in Mobile and past games when you played the tiny map 5v5 or 10v10 you picked your gun loadout, and the more you played the more XP it gained and you could start adding attachments, ect... but I can't seem to do that in MW since there is only BR that I can play, so do I HAVE to buy the battle pass, or the main game like black ops, MWII , MWII, ect... ????

next question, after an update, I go in game and it is loading Preshaders,, is there a way to see where it is and make sure it is loading up? I saw at the top at one point it said 83%,, but for 30 min it never moved.

Thanks from a Noob in advance.