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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/
I highly doubt card counting is easier in online casinos (unless you are playing blackjack online with live dealers).
You can't be sure if the card you counted is the same deck that you are playing to your current game in online casino which I highly doubt that it is easier than the physical casino. But yes, card counting is easier in live dealers coz you can basically have just a paper and pen on your side to list all the outs and possible next draw card. It is cheating but the casino won't see you lol...
Card counting is cheating in physical casinos, however I don't see it on TOS of any casino, nor being discussed in several threads.