How does the mvcc / row versioning changes that accumulate get shared among multiple processes? Is it another file like the WAL file? Or is it just local memory

How does the mvcc / row versioning changes that accumulate get shared among multiple processes? Is it another file like the WAL file? Or is it just local memory as part of the client? I guess with everything having to commit or entirely roll back at the end, it doesn't need to be shared. How does this compare with hctree then? It sounds similar, both using an MVCC model. Really cool though, love the technical details and would be interested to see further. I feel like there is certainly a space for sqlite+fancier features. Though it would break backwards compatibility, I've always wondered what kind of performance gains could be had by using a different varint implementation. And enforcing or always using "strict" for smarter column layouts. Also would be really cool to have indexed views. And columnstore seems like a logical direction as well.