Considering clusters need a load balancer in front of them, I'm going to say only through add-ons.
You can shard a table on the same server (it's called partitioning).
Not something I have done, haven't needed to yet.
Beyond that, it's pgcat I believe, in which query sharding is still experimental.
HAProxy and PGBouncer can do load balancing for reads, but ultimately the cluster is single main for writes.
Supabases Supavisor is another beta release option for postgres connection routing, but it is VERY early in development. I think they are dog-fooding it tho. Certainly one I am keeping an eye on!
However, that is my limited experience opinion.
Happy to be told otherwise, hopefully I start a discussion!