SSAA music

Take advantage of features
https://musescore.org/en/handbook/3/create-new-score#user-templates-fol…
https://musescore.org/en/handbook/3/create-new-score#add-instruments
https://musescore.org/en/handbook/3/copy-and-paste

There's no template for SSAA, but it should be easy to create one.
And yes, you can copy/paste, but that often doesn't look like a real piano part then.

"After I fill in the vocal parts, can I create the piano part by copying and pasting, or do I need to re-enter all the notes by hand?"

Some users have found the Implode function very useful for this purpose. See the Handbook:
https://musescore.org/en/handbook/3/tools#implode