When you get new game on steam there is some game drop that you get them by playing them and you can use them to level up your steam profile or sell them on community market but there will be never full card set so if you want to level up you need to buy remaining cards on community market
You need to have add friend on your profile it mean's to have 5 usd on your account

To get them fast and not to play that game you just need 1 app "Idle-master[github.com]" its so simple and safe to use no vac ban or stuff like that

1- Run steam
2- run idle master

3- sing in into your account

4- Then it'll be like this screenshot and you need to wait for few second

5-Now you need to just wait for some hours it depend how many cards you got left