When you play blackjack online for real money you play by the same rules as when you play blackjack in a brick and mortar casino. In fact, online casinos allow you to play alongside other real players from around the globe, allowing you to feel the same social aspect of the game as when you play in a live casino.
However, online casinos aren't able to physically stop you from applying card counting methods to your gameplay. In land-based casinos, players are able to be ejected for using such strategies, and obviously, you're a lot more visible in them. However, not all online casinos provide the ability to incorporate such methods into gameplay, as there are some platforms whose cards are shuffled after every hand, making card counting impossible. Apart from this, online blackjack allows you to go one-on-one with the dealer, no other players involved. More hands can be played per hour because of this. And finally, you can place large betting spreads without invoking any kind of suspicion.