How do those collaborative whiteboards work?

I'm talking about those online things like Miro, Figma's FigJam, Google's Jamboard. They are online collaborative whiteboards.

I'd like to build one for learning purposes but not sure what sort of tech stacks those things usually use.

I have some familiarity with React and websockets..is that what they are usually built with?

If anyone has some insight into those things I'd love to know!