Free games don't drop cards, if they did the cards wouldn't be worth anything would they.You only get cards from free games when you spend money on in-app purchasese, DLCs, micro-transactions etc - and you have to spend a lot for every single card drop.

sometimes games with cards are given for free by developers / publishers:

https://psteamcommunity.yuanyoumao.com/groups/freegamesfinders

hitman 2 prologue is completely free and you can idle it directly

For most games that have Steam trading cards, you earn cards by purchasing and playing the game, for which you will receive 1/2 the cards in the game's set (rounded up).

Free-to-Play games operate differently. You instead earn card drops by spending money, either from that game's in-game shop or by buying the game's DLC in the Steam Store. The current rate of all Free-to-Play games is approximately $9 US per card drop, but it is believed that Valve can adjust the rate on a per-game basis (though no games currently appear to be using any other rate).