user-branch-role-permission.php

<?php
$user = User::find($user_id);

$user->branches->load([
    'roles' => function ($q) use ($user_id) {
        $q->where('permissions_users.user_id', $user_id);
    },
    'roles.permissions',
]);

$user->branches->first()->roles->pluck('permissions')->collapse()->unique(); // 1 branch

$user->branches->pluck('roles.*.permissions')->collapse()->unique(); // all branches