Most letters and symbols that are common in the English speaking world fit into a single char, so pretending that a char is always "a single letter or symbol" is generally a good enough mental model.

Where this falls apart is with things like emoji (👨‍🍳) which are generally considered to be one symbol, but cannot be represented in a single char.

char chef = '👨‍🍳';

chars are actually "utf-16 code units". Many symbols require multiple "code units" to represent.

For a full explanation, refer to this old Computerphile video.

It describes "utf-8", which is 8 bits per "code unit." Java's char uses 16 bits, but that is the only difference.