How to redirect all requests within a subfolder

I have a website called mydumbwebsite.com/. In my root folder, I have various folders, one of them being "stuff". This folder is directly accessible with mydumbwebsite.com/stuff/. I want to create an .htaccess file that redirects everything that goes into this subfolder and only this subfolder, so these urls:

1. mydumbwebsite.com/stuff/test
2. mydumbwebsite.com/stuff/test/
3. mydumbwebsite.com/stuff/test.php
4. mydumbwebsite.com/stuff/test.php?test=yes
5. mydumbwebsite.com/stuff/test2.php (doesn't exist)
6. mydumbwebsite.com/stuff/test2.php?test=yes (doesn't exist)
7. mydumbwebsite.com/stuff/test/moar
8. mydumbwebsite.com/stuff/test/moar/tests.php

... should all redirect to the index.php file of the folder "stuff", even if the file/folder they point to exists.

Some additional context: despite my made up example, I encountered this problem on localhost. I have many different projects and I don't want the .htaccess of one project interfere with the other projects. I tried this, but it keeps redirecting me to the xampp homepage:

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

I tried changing it to this, but that couldn't cover all instances, such as nr. 2, 3 and 4:

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

from How to redirect all requests within a subfolder