Ok /wsr/

so instead of downloading CS:GO with my shitty internet i instead decided to ask someone with CS:GO. to go to the directory steam > steamapps > commons and to copy Counter-Strike Global Offensive to a USB.

now after the file was copied i put it into my steam directory. but whenever i try to open the game it simply installs the game for scratch even tho all the files are present. furthermore after i finally installed the game once (i actually do own the game) it simply deleted CS:GO from my PC entirely and forced me to restart the download.

how would i be able to get my local copy of CS:GO working and recognised by steam?

any help would be appreciated thanks