Handling query params with htaccess rewrites and simple php switch case?

Question

I am new to php stuff, so the question might be a bit stupid:

index.php

<?php

include "rest/db.php" ;
$db = OpenCon();

$request = $_SERVER['REQUEST_URI'];

switch ($request) {
    case '/' :
        require __DIR__ . '/home.php';
        break;
    case '/supply' :
        require __DIR__ . '/supply.php';
        break;
    case '/about' :
        require __DIR__ . '/about.php';
        break;
    case '/careers' :
        require __DIR__ . '/careers.php';
        break;

    ......//some more stuff

    default:
        http_response_code(404);
        require __DIR__ . '/404-page.php';

.htaccess

RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.+)$ index.php [QSA,L]

1 question:
I need to handle request to supply.php?page=5(or supply.php?page=5&offset=5). How do I need to change index.php and htaccess for that?

2 question:
I have admin template written in angular and hosted on the same hosting as this website(which is in php). Angular admin project is in /adminv1 folder. The problem with it is that, when I do refresh the page, it goes to 404 page and loses the styles. I guess this is happening because of the request handling in index.php. What would be the simplest way to solve it?

admin panel working

admin panel after refresh

Thanks in advance!