Big Sur related app updates on Catalina

10.13.5? Did you mean 10.3.5, because that's the latest I see on the Mac App Store. If that's the case then great, as it seems to look still like Catalina design, not having UI changed to Big Sur style like this:

How exactly the GarageBand update won’t install in Catalina? Like it won’t just let install or it’ll, but actually changes nothing or...

About the rest, does it mean that iWork apps like Pages would get Big Sur like UI change even on Catalina?

Icons having changed slightly means they’re now in those rounded rectangles?

Haha, understandable.

So it seems like those design changes only come when you already have some sorta design package that comes with Big Sur installed, as well as I could explain it. So I on Catalina not having those should still have older look. Icons I could just copy before updating and paste later.

Yeah, thanks, but I was already answered. That's great to hear anyways that apps, which are built with UI Kit elements or however to call it, won't be changed UI wise on older versions.

Why is GarageBand now Big Sur exclusive though?

It won't install

Right.

I don't use them enough to notice

Here are some screenshots for comparison.

Yep

Well, no issues with that really. Can just manually change them with old ones myself.