Hi there! 
Good questions both of these are very common menu-layout tweaks in Shopify themes.
Evenly spacing the top-level menu items (space-between)
This is usually controlled by CSS, not by a Shopify setting.
In most themes, the main menu is wrapped in a flex container. To evenly space items, you’d typically need something like:
.header__inline-menu ul {
display: flex;
justify-content: space-between;
width: 100%;
}
Depending on your theme, the class names may be different (for example: .header-menu, .nav-menu, etc.), so you’ll want to inspect the menu in your browser to confirm the correct selector.
You can add this in:
Online Store → Themes → … → Edit code → base.css / theme.css
Aligning dropdown (nested) menus under their parent item
By default, many themes position dropdowns relative to the whole header instead of the individual menu item.
To make each dropdown appear directly under its parent, the parent menu item usually needs:
.menu-item {
position: relative;
}
And the dropdown:
.submenu {
position: absolute;
left: 0;
top: 100%;
}
Again, class names will vary by theme, but the key idea is:
Important note
Not all Shopify themes are structured the same way, and some hard-code spacing or dropdown positioning in JavaScript. If CSS changes don’t work right away, it’s likely theme-specific.
If you share:
It’ll be much easier to give you exact selectors or a copy-paste-ready solution.