If you are using public transport in London, you can use an Oyster card or a contactless credit or debit card. Yes, you can also use mobile devices with eg Apple Pay.
You often see people putting their wallets directly onto the readers. If these contain more than one card that could be used to pay for travel, it is possible that the wrong card is chosen by the machine, or no card or all cards are used. This is the so-called card clash.
Do avoid it by carefully taking your card out before using it.