How to upload multiplayer game to STEAM ?

Hi devs, i am quite new to Subsystem stuff , what i want is i got a multiplayer game which rn works fine as LAN but only locally in the editor , waht i got to do to upload this game to Steam and then to be able to play with my friends from different PC as a Counter-Strike or PUBG ? I know that i got pay 100$ to upload to steam and thats the only stuff i know, i have read that there is stuff to do with handling Sessions can you explain more detailed about all this proccesses ?

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