No guaranteed order, but very typically a predictable order depending on the query plan chosen. If you’re relying on an ordering, you need to specify it in the query.

However, the scan order is predictable for a given engine depending on the query plan. If it’s doing an index scan, you’ll get results in index order and if it’s doing a table scan you’ll get it in table storage order (assuming a transactional, single node [edit: also single threaded] OLTP database- this goes out the window when outputs are aggregated across multi node (edit: or multi threaded) scans without an ORDER BY)

Postgres can do parallel table and index scans. Don't rely on order without explicit order by.

True, I forgot to consider how single node (multi threaded) parallelism can look a lot like multi node parallelism