htaccess for Angular app in a subfolder
You need to add the following rewrite in your Drupal .htaccess

RewriteCond %{REQUEST_URI} !/APP-FOLDER-NAME/.*


Then in the root of the angular application folder you need the following in your .htaccess

RewriteEngine on
RewriteCond %{REQUEST_FILENAME} -s [OR]
RewriteCond %{REQUEST_FILENAME} -l [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^.*$ - [NC,L]

RewriteRule ^(.*) /APP-FOLDER-NAME/index.html [NC,L]