I really hope it's not the way I think it is, but as I understand, it randomizes from the cards you already have, and as a newbie - the only cards you have are the default one's, which quite frankly, makes your deck far from being good (at best it's passable, possible to beat easy mode atleast). Not really a great thing, I'll be honest, making it so random cards actually just gives you fully random deck, even cards that you don't have, is a much more interesting and engaging mechanic. Pretty much let's you play this as a roguelike, which makes replayability sky-rocket.

After a certain period, you will be locked with a limited amount of decks. I think the trial period only lasts of 2 days. After that, you won't be able to play all the cards cuz the devs (who already abandoned this game) changed it several years back.