That depends on your setup, for the master instance you'd run it just as you would for other setups. Streaming replication is common there. For the sharded instances, Citus has the ability for you to set what your replication factor is. Here Citus is then aware of when a node fails and will automatically redistribute the data to a new node, essentially taking care of that for you.
Since the move to open source, more recent upstream changes have been slowly merged in the code base, though they seem to be still on a 8.3 base, still a couple of years worth of code to go through.
Greenplum is a fork of Postgres codebase, Citus is not; it's an extension that leverages community Postgres's extensibilty APIs. This point seems to be highlighted in their post.