Is it possible to have multiple instances of one shape that are all updated once you have changed the original shape?
Using symbols is aready written.
There's still left another way: Placed Affinity documents are linked copies which get changed if one edits the original. It's made easy. One can take the original under edit wth a double click.
NOTE: Placing takes whole document, also the otherwise empty page. Fortunately it's transparent if not otherwise specified.