Page Tabs
You can add your own tabs to any of the Admin UI's list or detail pages using the registerPageTab function. For example, to add a new tab to the product detail page for displaying product reviews:
src/plugins/reviews/ui/providers.ts
import { registerPageTab } from '@vendure/admin-ui/core';
import { ReviewListComponent } from './components/review-list/review-list.component';
export default [
    registerPageTab({
        location: 'product-detail',
        tab: 'Reviews',
        route: 'reviews',
        tabIcon: 'star',
        component: ReviewListComponent,
    }),
];

If you want to add page tabs to a custom admin page, specify the locationId property:
src/plugins/my-plugin/ui/routes.ts
import { registerRouteComponent } from '@vendure/admin-ui/core';
import { TestComponent } from './components/test/test.component';
export default [
    registerRouteComponent({
        component: TestComponent,
        title: 'Test',
        locationId: 'my-location-id'
    }),
];
note
Currently it is only possible to define new tabs using Angular components.