In WordPress, users are typically assigned a single role, such as Administrator, Editor, or Subscriber. However there are situations where a user might need multiple roles. For example, you might want a user to have the permissions of both an Editor and a Contributor, or a Shop Manager and an Author for a more complex content management system.
Add the Ability to Select Multiple Roles in the User Profile:
We’ll begin by adding a multi-select drop-down to the user profile page in the WordPress admin dashboard, where administrators can select multiple roles for a user.
Code to Add Roles Selection Field:

function allow_multiple_roles($user){
    if(current_user_can('edit_users')) {
        $all_roles = wp_roles()->roles; //Get all available roles
        $user_roles = $user->roles; //Get current user's role
        ?>
        <h3><?php _e('Multiple Roles', your_text_domain); ?></h3>
        <table class="form-table">
            <tr>
                <th><label for="multiple_roles"><?php _e('Assign Multiple Roles', your_text_domain); ?></label></th>
                <td>
                    <select name="multiple_roles[]" id="multiple_roles" multiple="multiple" class="regular-text" style="width: 200px;">
                        <?php foreach ($all_roles as $role_slug => $role_details): ?>
                            <option value="<?php echo esc_attr($role_slug) ?>" <?php echo in_array($role_slug, $user_roles) ? 'selected':''; ?>>
                                <?php echo esc_html($role_details['name']); ?>
                            </option>
                            <?php endforeach; ?>
                    </select>
                    <p class="description"><?php _e('Select multiple roles for this user.', your_text_domain); ?></p>
                </td>
            </tr>
        </table>
        <?php
    }
}
//Add the field to the user profile pages
add_action('show_user_profile', 'allow_multiple_roles');
add_action('edit_user_profile', 'allow_multiple_roles');

The allow_multiple_roles() function adds a multi-select dropdown field on the user profile page.
The dropdown is populated with all available roles in WordPress (wp_roles()->roles).
If the user already has a particular role, it will be pre-selected in the dropdown.

Save the Selected Roles:
This part of the code will save the selected roles when the user profile is updated.

function save_multiple_roles($user_id) {
    // Make sure the current user has permission to edit this user
    if(!current_user_can('edit_user', $user_id)) {
        return false;
    }

    //Check if the 'multiple_roles' field is set and is an array
    if(isset($_POST['multiple_roles']) && is_array($_POST['multiple_roles'])) {
        $roles = $_POST['multiple_roles'];

        //Get the user object
        $user = new WP_User($user_id);

        //Remove all current roles
        $user->set_role('');
        
        //Add selected roles
        foreach ($roles as $role) {
            //Validate the role before adding
            if(array_key_exists($role, wp_roles()->roles)) {
                $user->add_role($role);
            }
        }
    }
}

// Hook the saving process to the 'profile update' action
add_action('profile_update', 'save_multiple_roles');

The save_multiple_roles() function first checks if the user has permission to edit the profile.
It then looks for the multiple_roles field and processes it as an array.
Before adding the new roles, the user’s existing roles are removed with $user->set_role('').
The selected roles are validated then added to the user using $user->add_role($role).

After adding the code to your theme’s functions.php or a custom plugin, you can test it by following steps:
Go to User Profile: Navigate to Users>All Users and click on the “Edit” link of a user.
Select Multiple Roles: In user’s profile page, you’ll now see a multi-select-dropdown labeled “Assign Multiple Roles”. Select the roles you want to assign to the user.
Save Changes: After making the selections, click the “Update User” button.
Check the User’s Roles: To ensure that the roles were assigned correctly, you can revisit the user’s profile and confirm that the roles are displayed in the multi-select field.