How can two people work on one file at the same time? [on hold]

I recently read blockchain technology introduction lines and I thought it was interesting, but implemented in a web way, like Google Docs did. My question here is, from where I can start reading on how to implement such a collaboration as in Google Docs? Like, they know what line you are editing right now in the document, or if u are in the document at all. I am not limited to technologies, just want it to be web. Any ideas are appreciated.