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 think it's very hard to card count in black jack these days. Going to a physical casino alone without a team is almost impossible. It is very suspicious if you are standing behind a table and just count the deck for 20 minutes and then suddenly jump on the table at start betting the maximum amount. Security will be pretty much be on top of you within the first hour of gambling and you just end up being banned from the casino. With a group of people - multiple spotter - at least 2 gamblers - 1-2 decoys - and a good exit strategy it might be possible.