How do I open old Notes saved on my external hard drive? I've located the files in:

~/Library/Group Containers/group.com.apple.notes/

but now I am unsure how to actually open them and view the content. "Open With" Notes app does not seem to work
You could use an app like https://www.sqlitepro.com/ to open the SQLlite database. But are you sure your data is there? At my system there is nothing in it.