Weel thats an actually an interesting question, never though about that.
But from my limited knowledge of RNG which is mostly java.util.random I would guess that there is no shuffling involved at all, you just got random card from the deck then another random card from the remaining part of the deck then same goes with flop turn and the river. Tho it could be setup to shuffle the deck before and make it an array before the hand starts.
Whatever it is, it wont change anything from player's perspective imo, its just a system solution what to use...