Basics of overriding an admin page template - Craft 3

Question

I want to override the admin/users page template when a plug in is installed.

Detail: On the user page domain.com/admin/user all the groups are listed in the sidebar area. I get this from the \vender\craftcms\cms\src\templates_layouts\cp.html file. I want to install/use use a custom template for the user page using a plug in.

What are the basics of overriding a admin template through a plugin?

Edit - More detail:

I have a plugin that creates a new controller exactly the way that is proposed in this post. The controller does two things (maybe more later):

1) On entries page it displays all the entries to a logged in user that was posted by them and others from their same group. It does not display entries that were posted by users in other groups.

2) On user page it displays only users from their own group. It does not display any other users from other groups.

On the users page all the groups are listed here:

It does not make sense to display all the groups here as the logged in user can only see or edit users in their own group.

I need to change what is displayed in the group listing area.

Answer 1

There are multiple ways to achieve your goal, I think the most elegant one is to overwrite the default route /users with your custom one and replace the elementType

Things you need to do

1. Change the default route to your template
2. Copy the default template and change it's element type
3. Create a custom element type that extends craft/elements/User

You can change the route of all navigation bar items (modifyCpNav)

Event::on(
    Cp::class, 
    Cp::EVENT_REGISTER_CP_NAV_ITEMS, 
    function(RegisterCpNavItemsEvent $event){
        // the 4th element is the "User", you can change the URL to your
        // custom template instead of the default template
        // I suggest you to loop all `navItems` and find 
        // the correct user instead of assuming it's the 4th
        // Craft::dd($event->navItems);
        $event->navItems[4]['url'] = 'path/to/template/user';
});

Then just copy the Craft templates/user/index template in your plugin to create an index page

{% if CraftEdition != CraftPro %}
    {% exit 404 %}
{% endif %}

{% requirePermission 'editUsers' %}

{% extends "_layouts/elementindex" %}
{% set title = "Users"|t('app') %}
// changes start
// replace the elementType with your custom element type
{% set elementType = 'prefix\\namespace\\elements\\User' %}
// changes end
{% block actionButton %}
    {% if currentUser.can('registerUsers') %}
        <a class="btn submit add icon" href="{{ url('users/new') }}">{{ "New user"|t('app') }}</a>
    {% endif %}
{% endblock %}

Then in your src/elements folder create a new User class to overwrite the sources

class User extends \craft\elements\User{ //<-- extend the default User element

    /**
     * @inheritdoc
     */
    protected static function defineSources(string $context = null): array
    {
        $sources = [
            [
                'key' => '*',
                'label' => Craft::t('app', 'All users'),
                'criteria' => ['status' => null],
                'hasThumbs' => true
            ]
        ];

        // Overwrite the sources / usergroups...
        // usually Craft will search for usergroups at this place..
        // just leave it empty, so you'll only see the "default" group

        return $sources;
    }
}

Don't forget to register your element

TL/DR
You could as well just hide the other sources via CSS/Javascript - but I would not suggest it, since everyone is able to see the other groups with minor Javascript skills