someone talk me through my inventory and load out... what to do, not to do... what it's for etc etc
Only real changes you can make are for CTs:
M4A1s vs M4A4:
The M4A1s is silenced so enemies don't see the tracers when you spray through smoke (sothey don't know where you're shooting from and instead have to guess). However the M4A1s only has a 20 round magazine with 40 rounds on your belt. So if you're not careful you can run out of rounds. M4A1s has a tighter spread when you spray or burst so it is more accurate. It's fire rate is lower though so overall it could be argued that it does less damage. Burst fire and being sneaky is how you want to use this gun.
M4A4 by contrast is good for spraying (30 round magazine with 60 in the belt), has a high rate of fire but a bigger spread. Spray pattern is very similar to the AK so if you can shoot the AK you'll lean the M4A4 fast.
So which you choose depends on your play style. I prefer the M4A1s but there are pros and cons for both. If you're a lurker or you like holding and playing slow with burst or tap fire go for the M4A1s. If you're an entry fragger or enjoy spraying then go for the M4A4.
CZ-75 vs five-seven:
I'd recommend the five-seven. The cz-75 is fully automatic but only has 24 rounds. You can burn through those rounds in seconds so trigger discipline is very important. You also only get $300 for a kill with it, like killing someone using a rifle. So it won't net you much income on eco rounds. The cz-75 is good for 1v1 fights against a guy with a rifle close range. You are very likely to end up killing him and stealing his weapon.
The five-seven by contrast holds 20 rounds in the magazine and has 100 rounds in your belt. It is a 1-shot headshot close range and surprisingly accurate while moving. Long range it is quite garbage though so you'll want your enemy to be close.
Overall the five-seven is more versatile. I'd recommend it over the cz-75.
USPS-S vs. P2000:
Both are pretty similar. P2000s spray pattern tends to go vertical while the USP-S goes sideways slightly. None of thse matters though because you shouldn't be spraying with these guns. Take it slow and aim. USP-S has a silencer so, as mentioned previously, the tracers can't be seen through smoke.
USe the one that feels most comfortable for you. I use the USP-S because I find it more accuaret for long to medium range headshots (which on pistol rounds is what you want). However if you tend to burn through ammo use the P2000 as you get far more rounds with it than with the USP-S.
On terrorists:
cz-75 vs. Tec-9:
Similar story to the five-seven vs cz75. Use the Tec-9. It holds 24 rounds. It almost doesn't give a siht about armour. It fires so fast while maintaining accuracy that it may as well be an SMG. It has a disgusting amount of ammo on your belt, you will never run out.
One both sides:
R8 vs. desert eagle:
R8 is super accurate long range. This is down to preference I suppose. Most people use the desert eagle because it is usually a 2-hit to the chest kill and a 1-shot headshot kill but the deagle fires quicker. I don't really use these weapons in competitive so I just stick with the desert eagle.