In this brawl, if you don't fill your deck then the game gives you random cards from the 3 expansions and since the cards are random you can even get 10x the same card. (But that would be super rare ofc)
You can add any number of cards (less than 30) to your brawl deck and still get added random cards, but I don't know the logic how your deck is autofilled other than you don't need to own the cards you get in your deck. I got many random legendaries added to my deck even though I don't even have most of them in my collection.
Also the autofill doesn't always reach a full deck of 30 cards, meaning you might add 11 cards to your deck and start the game with only 26 cards. Don't ask why. I played with DH without adding any cards myself and got a deck of 12 cards or something like that lol