Livebook actually uses state reducers as described here[1]. Except for cell content. There it uses Operational Transformation. The source code[2] is really instructional and easy to understand. I highly recommend to look into it :)

[1] https://github.com/jonatanklosko/notebooks/blob/main/article... [2] https://github.com/livebook-dev/livebook/blob/main/lib/liveb...