Nothing to do with plan caching, thats just talking about plan execution of parallel operations which is that thread or process based in PG?
If process based then they can send small parts of plan across processes.
If process based then they can send small parts of plan across processes.
Plans for prepared statements are cached though.
Other databases like MSSQL have prepared statements but they are rarely used now days since plan caching based on query text was introduced decades ago.