copy source destination
The pattern of git push command
push destination source:destination
the first destination is the destination repository - the second is the branch (and 'source' is the source branch). I must have skimmed the linked manual dozens of times over many years. There was something that just did not make sense and I imagined that I would have to read some book or something to eventually understand the magic 'ref specs'. It never dawned on me that it was that simple.
The additional difficulty is that the destination repository is usually shortened to 'origin' (because it was the origin for the clone command).