In my opinion, card counting is applicable to online and physical casinos. I think iit works better in online casinos because there is nothing the house can do. There are other solutions and the solutions above are applicable mostly in physical casinos.
How to count cards in blackjack link:https://www.blackjackapprenticeship.com/how-to-count-cards/
Card counting is not really applicable online unless you are playing with a live dealer otherwise each time a new round begins the deck gets shuffled completely giving you no chance to use card counting at all, this is important to know since you do not want to apply card counting in those instances and you should just use the basic strategy and nothing else, however even when using this with a live dealer on an online casino the casino can simply cancel your account if they detect any indication you are using card counting, in a physical casino you could always get away with it if the dealer forgets the count or if you give enough good tips that he decides to ignore your profits that day, but in an online casino everything you do is recorded and a research on your betting patterns will reveal you as a card counter immediately.