Learn how to make a php script(app) accessible to all wordpress subdomains in a wordpress multisite configuration?

i’ve a wordpress 5.8.1 set up on Home windows 10 with a multisite configuration regionally (localhost) and that i need to entry a php app in an app folder from all subdomains i created. (There are three at whole)
e.g:
localhost/site1
localhost/site2
localhost/site3

What i need to do is for each website (site1,site2,site3) is that they’ll entry the app folder
e.g:

localhost/site1/app
localhost/site2/app
localhost/site3/app

How do i procede to take action? Ought to i put the app folder at wordpress’ root folder or one other folder? And what ought to i accomplish that that each subdomain has entry to the app folder? I actually do not know learn how to procede in order that it really works like i would like. If anybody can level me to the suitable path this could assist me very a lot.

this is what i added to my default wordpress wp-config file:

outline('WP_ALLOW_MULTISITE', true);
outline( 'MULTISITE', true );
outline( 'SUBDOMAIN_INSTALL', false );
outline( 'DOMAIN_CURRENT_SITE', 'localhost' );
outline( 'PATH_CURRENT_SITE', '/wordpress/' );
outline( 'SITE_ID_CURRENT_SITE', 1 );
outline( 'BLOG_ID_CURRENT_SITE', 1 );

and this is my .htaccess file:

# BEGIN s2Member GZIP exclusions
<IfModule rewrite_module>
    RewriteEngine On
    RewriteBase /wordpress/
    RewriteCond %{QUERY_STRING} (^|?|&)s2member_file_download=.+ [OR]
    RewriteCond %{QUERY_STRING} (^|?|&)no-gzip=1
    RewriteRule .* - [E=no-gzip:1]
</IfModule>
# END s2Member GZIP exclusions

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
RewriteBase /wordpress/
RewriteRule ^index.php$ - [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content material|admin|contains).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*.php)$ $2 [L]
RewriteRule . index.php [L]
</IfModule>

# END WordPress

I hope that i am heading in the right direction, if anybody may help me this could be good.