The Office 365 backend uses git to store snapshots of documents. [1]
https://github.com/microsoft/FluidFramework/tree/main/server...
That is not exactly what it says. It says it uses a content-adressable system and you can talk to it as if it was git.
And the content-adressable filesystem that it uses is git [1]. It uses git with a javascript implementation of it [2] or node bindings to a c one [3].
1. https://github.com/microsoft/FluidFramework/tree/main/server...