I read about android single source of truth to provide offline data. But how to deal with post, patch request?

That topic is way too broad to be answered here and much of the work has to happen on the back end side. You can Google for a comprehensive series of blog posts by Trello. Some additional keywords: CQRS, event sourcing

Thanks

what about post requests?

You need to reconcile conflicting updates if the data is not read-only. You can't even easily say which update came first because the participating systems may not have the same understanding of time. A classical distributed systems problem.