Help with nav menu

I’m definitely a novice, as I’m a volunter trying to improve a non-profit’s website. I’m using Twenty Twenty One, Classic editor (I’m too afraid to try and switch it to Block editor). As I’m slowly learning some stuff – I’m not a techie – I’ve been learning by trial and error (mostly error), and I somtimes screw things up as I’m trying to grow the website and improve it’s appearance! In messing with the theme’s Nav Options, coupled with messing with CSS, and adding submenus, I’ve managed to create some problems with the Nav menu:

1. On each page, the menu starts out in the header, but as one scrolls down, it jumps to the upper right corner. Is there a way to keep it consistantly in one place? The menu appearance changes after it jumps to the corner: the submenus are no longer compressed; it sits on a white background; and it moves down the page, divorced from the “X” closer. How can I keep it consistant as it appears when one first lands on a page? i would also like it to have less blank green space on the right side
2. I’ve also done something to the nav menu on mobile devices by (I think) not fully understanding how to define the breakpoint between big and small screens and set the proper width for the menu so it works both on big and small screens. Can submenus be compressed on small screens?

I would greatly appreciate any help to fix my screwups and get this site running properly again.

The page I need help with: [log in to see the link]