BuddyPress.org

“The default theme isn’t intended to be used on hosted blogs on a WPMU install, so the links do break as you’ve found.”

Oh… I see. Thank you! With that information, I have now found this and will give it a shot (I guess it’s obvious, but I was just confused about how it was supposed to work):

https://codex.buddypress.org/how-to-guides/building-a-buddypress-child-theme/