New to UE5, why does this happen with the viewport?

Hey!
I asked a question a couple of days ago about how to make the UI interactable, and after your guys’ help I’ve managed to make it work!
Been putting in a bunch of effort today and I’m very happy with my progress, but, one thing I can’t figure out (even after googling a lot) is why the viewport doesn’t work the way I want it to?
Please look at the pictures I’ve attached.
I have a widescreen monitor, so I think that changes the viewport default to a widescreen, but I’ve tried changing a bunch of settings to make the viewport 1920x1080 (which is what I want for this project) but I can’t do it. The viewport doesn’t change.
Now I made a new menu, and I see that the new menu, for some reason, covers the entire screen even though the previous ones don’t. Why does this happen?
Thanks.

On the main menu, you can see that the menu doesnt cover the entire screen, which is good I guess because I want it to be in 1920x1080, which this seems to be, even though the viewport extends more than that. (So how can I make the ■■■■ viewport just show 1920x1080?)
The other picture (where it says “Pick Creator”), you can see that it’s still the way I want it, but again, the viewport for some reason extends there to the right.
The last image, where you can select one of the characters (without it saying “Pick Creator”), you can now see that for some weird reason the Background image now extends throughout the entire viewport.

What gives? And thank you guys for your help, arguably the most helpful community I’ve ever seen. Good stuff.