One of the ways in which online blackjack is different from playing at a brick and mortar casino is how the cards are dealt. In a brick and mortar casino, there is a croupier at the table who deals the cards to each player and shuffles the cards by hand, or using a shoe or other shuffling device. That shuffling ensures the randomness of the cards so everyone has an equal chance of winning.
With online casinos, there is no dealer and, technically speaking, there are no cards. There are only digital representations of cards that are given to you and the dealer. Instead of the decks being shuffled, the randomness is guaranteed by a computer program called a random number generator, sometimes called RNG for short.
A random number generator uses a complex algorithm to determine the outcome of which cards you are dealt. Simply put, the RNG produces a set of numbers hundreds of times every second, including when the game isn’t being played. Each set of numbers corresponds to a certain card. When you press the button to take a card determines what card you are dealt. Whatever number is selected by the RNG at that exact microsecond determines your card. The same goes for the dealer. Therefore, you can never accuse someone else of taking your card, because it would not have been your card.
The random number generator makes the card you are dealt impossible to predict, including by counting cards. When blackjack is played using a deck or multiple decks of cards, card counters can determine how likely they are to receive a 10-value card based on how many of those cards have been dealt already. With online blackjack, however, predicting your cards by counting them is impossible, because the RNG essentially acts as if the deck is being shuffled before every card is dealt.
So if you’re trying to count cards online, stop. It does no good. Instead, accept the fact that you can’t predict what card you or the dealer will be dealt (including the dealer’s hole card) and stick to blackjack basic strategy, which can reduce the house edge to 0.5%.