Source: https://divibooster.com/add-a-separator-between-the-divi-primary-menu-items/

Customizer menu

/* START add separators between navigation menu items */
.et_header_style_left #top-menu > .menu-item + .menu-item:before,
.et_header_style_centered #top-menu > .menu-item + .menu-item:before,
.et_header_style_split #top-menu > .menu-item + .menu-item:before
{
	position: absolute;
	left: -15px; /* Half of default 22px right padding and 4px adjust to center (to compensate for space char between li) */
	content: '|';
	font-size: smaller;
	color: #000000;
}

/* Handle positioning for mega menus */
.et_header_style_left #top-menu > .menu-item + .mega-menu:before {
	position: relative;
	top: 1.1em;
}
/* END add separators between navigation menu items */Code language: CSS (css)

Menu module

https://divibooster.com/add-vertical-separator-bars-to-the-divi-menu-module/

/* START add separators between navigation menu items */
.et_pb_menu .et-menu.nav > li + li > a:before {
	position: absolute;
	left: -11px;
	transform: translateX(-50%);
	content: '|';
	font-size: smaller;
}
/* END add separators between navigation menu items */Code language: CSS (css)