Welcome to Android Central! See if this helps:
http://www.samsung.com/in/support/mo...-new-features/
For your questions:
1. You can toggle that Google Discover page to the left of the main homescreen on or off (and you might also have the option to change it to Samsung Free). Long-press any empty part of the homescreen, then swipe from left to right until you see the screen to the left of the main homescreen. There should be a toggle switch to turn that screen on or off, and possibly the options to select Google Discover or Samsung Free. (I'm basing this on my Galaxy A32 5G running One UI 3.1.)
2. You'd have to install a 3rd party launcher like Nova to get more options for grid sizes.
3. The Homescreen grid is how your app shortcuts will be set up on the homescreen. The Apps Screen grid is how the icons will be set up in the App Drawer (i.e., the screen that shows all of your installed apps when you swipe up from the bottom of the homescreen).