How real time collaborative editors store document in database ? What should be the schema to support "Operational Transform" or CRDT data structure?

Question

I am trying to understand the schema and right database for the real-time collaborative editor. I could not find a suitable article or research paper about database requirements so I am posting my question here.

It would be really helpful if someone can help me to understand. Or can provide a document or blog link.

Disclaimer: This is not a homework question :)