Could someone guide me through the use of symlinks in nginx for php modules?

so I'm running an ubuntu 20.04 server, with Nginx on it. After a difficult week, I managed to make it run normally, including php through php-fpm.

My problem is that I don't want hundreds of folders in my website's root (set at /var/www/www.my-domain.com/), so I'd like to put all the php related extensions I install (such as phpmyadmin, and IceCoder so far) in another folder (let's say /var/php/) and link all requests of php index from the root folder (like if I type https://my-domain.com/icecoder or https://my-domain.com/phpmyadmin) to redirect me to the folder where their index.php files are.

The problems with that are :

• first, I don't know how to make the symlink work (I know the "ln -s target destination" command) but it never works.
• second, that I don't know how to configure the default.conf file to make it happen.

I tried to pout a simple index.php with <?php phpinfo();?> inside in a /php folder in my root folder, but when I access it through the browser, I get an error "No file input setup". If I try to just access the said folder (for example https://my-domain.com/icecoder) I end up on my website, and for this one, the direct address to this index.php file works... except I only get a "working" state for hours, the text but no css. Dev tools do not even load.

Here are my config files (my real domain is anonymised under my-domain):

1. nginx.conf

    user www-data;
    worker_processes  auto;
   
    error_log  /var/log/nginx/error.log notice;
    pid        /var/run/nginx.pid;
   
   
    events {
        worker_connections  1024;
    }
   
   
    http {
        include       /etc/nginx/mime.types;
        #include      /etc/nginx/sites-available/*.conf;
        #default_type  application/octet-stream;
   
        #root       /var/www;
        log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
                          '$status $body_bytes_sent "$http_referer" '
                          '"$http_user_agent" "$http_x_forwarded_for"';
   
        access_log  /var/log/nginx/access.log  main;
   
        sendfile        on;
        #tcp_nopush     on;
   
        keepalive_timeout  65;
   
        #gzip  on;
   
        include /etc/nginx/conf.d/*.conf;
    }
   

2. default.conf

    server {
        server_name  my_domain.com;
   
        #access_log  /var/log/nginx/host.access.log  main;
        root /var/www/www.my_domain.com;
        index index.html index.php;
   
        location / {
            try_files $uri /index.html index.php =404;
        }
   
        # redirect server error pages to the static page /50x.html
        #
        error_page 500 502 503 504  /50x.html;
        location = /50x.html {
            root   /var/www/www.my_domain.com;
        }
   
        error_page 404 /40x.html;
        location = /40x.html {
            root   /var/www/www.my_domain.com;
        }
   
   
        # proxy the PHP scripts to Apache listening on 127.0.0.1:80
        #
        #location ~ \.php$ {
        #    proxy_pass   http://127.0.0.1;
        #}
   
        # pass the PHP scripts to FastCGI server listening on 127.0.0.1:9000
        #
        location ~ [^/]\.php(/|$) {
            fastcgi_pass unix:/var/run/php/php7.4-fpm.sock;
            fastcgi_split_path_info ^(.+\.php)(/.+)$;
            fastcgi_index index.php;
            fastcgi_param PHP_VALUE open_basedir="/var/www/www.my_domain.com/:/var/www/www.my_domain.com/php/";
            include fastcgi_params;
   
        }
        # SCRIPT_FILENAME parameter is used for PHP FPM determining
        #  the script name. If it is not set in fastcgi_params file,
        # i.e. /etc/nginx/fastcgi_params or in the parent contexts,
        # please comment off following line:
        #    fastcgi_param  SCRIPT_FILENAME  /scripts$fastcgi_script_name;
        location ~ /\. { # le "~" indique une regex que "/" démarre, ici tout ce qui a un "." autre que php ou html
            deny all;
            access_log off;
            log_not_found off;
        }
   
        listen 443 ssl; # managed by Certbot
        ssl_certificate /etc/letsencrypt/live/my_domain.com/fullchain.pem; # managed by Certbot
        ssl_certificate_key /etc/letsencrypt/live/my_domain.com/privkey.pem; # managed by Certbot
        include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
        ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot
   
   
    }
   
    server {
            listen 80;
            server_name www.my_domain.com;
   
            return 301 https://$server_name$request_uri; # redirets http requests to https website
    }
   
    server {
           listen 80;
           listen 443 ssl;
           server_name my_domain.com;
           return 301 http://www.my_domain.com$request_uri;
    }
   

here is an ls -l of the root folder

-rw-r--r--  1 www-data www-data   507 Sep 21 20:03 40x.html
-rw-r--r--  1 www-data www-data   507 Sep 21 20:04 50x.html
drwxr-xr-x  6 www-data www-data  4096 Aug 26 23:45 assets
-rw-r--r--  1 www-data www-data 83243 Aug 27 02:19 favicon.ico
-rw-r--r--  1 www-data www-data 25942 Aug 27 02:19 favicon.png
drwxr-xr-x 13 www-data www-data  4096 Sep 21 17:34 ICE
-rw-r--r--  1 www-data www-data 63175 Sep 21 15:58 index.html
-rw-r--r--  1 www-data www-data    19 Sep 20 22:24 index.php
drwxr-xr-x  2 www-data www-data  4096 Aug 27 00:31 maintenance
drwxr-xr-x 15 www-data www-data  4096 Sep 21 16:08 pages
lrwxrwxrwx  1 root     root        12 Sep 21 21:36 php -> /var/www/php
drwxr-xr-x  6 www-data www-data  4096 Sep  8 16:13 sections

and of the /var/php folder

drwxr-xr-x 13 www-data www-data 4096 Sep 21 17:34 icecoder
-rw-r--r--  1 www-data www-data   19 Sep 21 21:39 index.php
lrwxrwxrwx  1 root     root       12 Sep 21 21:48 php -> /var/www/php
drwxr-xr-x  2 www-data www-data 4096 Sep 21 21:37 phpmyadmin

All my folders are in 755 and files inside in 644 mod.

Thanks for any help you can give me