How to setup nginx on a subfolder

I bought a VPS running Ubuntu 20.04. I am fairly new to all the hosting and Linux stuff and I’m struggling on setting up the following situation or if this is even possible:

I have my /var/www folder which has many subfolders. Each holds a website. I want, if someone access my domain (e.g. mydomain.com) he should see the folder /var/www/codeiginiter4/. But when someone excess mydomain.com/foldername he should see the content of the /var/www/foldername/.

What makes it a little more tricky: CodeIgniter uses the following .htaccess for apache:

RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ index.php/$1 [L]

This applies the mydomain.com/foo to mydomain.com/index.php/foo for the CodeIgniter SEO friendly URL routing. For nginx they provide

location / {
    try_files $uri $uri/ /index.php$is_args$args;
}

I got it working with

location ~* ^/(assets|files|robots\.txt) { }

But I have the issue that whatever you add after mydomain.com/ will route to my CodeIgniter folder instead of other websites in other folders.

From my understanding try_files $uri $uri/ /index.php$is_args$args; will check $uri (a file), then a folder $uri/ first, so it should serve a folder if there is one, but I always end in my CodeIgniter page.