For the xbox one, how do i get the add-ons i want to buy for a favorite game? i see on the xbox's game pass page it shows "$9.99". i pay?

only with goft cards so my Q is: do i need to buy the "xbox game pass monthly" plan?
Update: i used an 'xbox' card to buy the game called "cities: skylines". what card by xbox did i buy and can i use that same one to buy "cities skylines's" add-ons? please let me know the answers to these questions soon
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