By default, the Divi Theme shows the same menu items in both the desktop and mobile versions of the header menu (albeit with different formatting). If you'd like to exclude certain items from showing up in the mobile menu, i.e. making them appear on the desktop menu only, here's how to do it.

Add a CSS class to the menu item

From your WordPress dashboard, go to "Appearance > Menus" and in the "Select a menu to edit:" dropdown, select the menu marked "(Primary Menu)". You should now see the menu items making up the menu in your header.

Now, at the top right of the screen you should see a "Screen Options" tab. Click this, and check the "CSS Classes" box, as shown here:

Now, in the Primary menu you just selected from the dropdown, locate the item you wish to hide. Click on it to expand its options and you should see a "CSS Classes (Optional)" field. Enter "desktop-menu-item" into this box. This will add a CSS class to the menu item which we'll use next to hide the items on mobile. If you want to hide more than one item, repeat the process for other items, entering the same CSS class (i.e. "desktop-menu-item") each time. Save the menu.

Hide the Menu Item(s)

Now, add the following CSS to your site:

Related Post: Adding CSS to the Divi Theme

You can add that CSS at the end of the "Divi > Theme Options > General > Custom CSS" box, or in the style.css file of a child theme.

Now when you view your site, the menu item(s) which have been assigned the CSS class should show on the desktop menu, but not in the mobile menu.