How is binary data 'split'?
It's a timing thing (I don't like the word 'just').

In the case of a UART, in which the A stands for Asynchronous, there is a line idle state, then a start bit, which edge the receiver uses to synchronise its timing. After that, all the bits are sent each with an equal period. The word finishes with a stop bit, which ensures there is a transition into any following start bit.