I would love to know how can i use this type of stuff in order for me to get better in the game.

you need to find doctor who can replace your limbs.

P.S.

Idk bag or feature but after i get 3-rd arm with Ensis mission i try to replace it with better one but my 3-rd arm just disappear and doctor make different operation... 

He replace my leg with arm so now i have 3 arms and 1 leg .  And 1 minor problem. Now i have only 1 slot for boot and 3 arm slots 

You go to a surgeon , and ask to "Replace Limb". (Cost around 550, depending at your "Influence" score.)

Or, if you have a surgeons kit, you can do the replacement yourself!

(It may be smart to remove armour & weapons from the limb before replacing it.)