UNION does not append

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/18/queries-union.html
Description:

I'm very confused about this statement in the documentation of UNION: "UNION
effectively appends the result of query2 to the result of query1".
Because, if I append one item to a list, it is still the same list. But in
the case of UNION, it's ""copied"" into a new anonymous derived-table. Alias
don't work anymore, which I would expect if I just append some rows. Or is
it like "effectively" means that this statement is not to be taken
literally? In that case I would really appreciate a more detailed
description, that after the UNION with tables a new anonymous derived-table
is returned and the original first table can't be accessed anymore. Thank
you for taking your time to read this even if you disagree.