Nginx reverse proxying to different root

Question

Took days to troubleshoot this issue, yet couldn't find solution googling with my noob knowledge base.

What I'm trying to do: Accessing url/nextcloud reverse proxy to localhost but keeping /nextcloud/ in user's view. i.e. url/nextcloud/login would internally be localhost/login in nextcloud configuration file.

Expectation: As 'nextcloud' config file change root from /var/www/ to /var/www/nextcloud, url/nextcloud/login would proxy-ed to localhost/login, in effect /var/www/nextcloud/login.

Result I always get: url/nextcloud returns url/login, aka url/var/www/login in effect rather than url/var/www/nextcloud/login

Below is my nginx reverse proxy code, and nextcloud code uses config in Nextcloud documentation. Reverse-proxy passes nextcloud to 127.0.0.1:5141 and Nextcloud configuration listens on 127.0.0.1:5141. Sorry I can't provide exact nextcloud config since my server went down today.

server {                                                                                             
        listen 80;
        listen [::]:80;                                                                              
        server_name MY_DOMAIN_HERE;                                                            
        # enforece https                                                                             
        return 301 https://$server_name:443$request_uri;                                             
}                                                                                                    

server {
        listen 443 ssl http2;                                                                        
        listen [::]:443 ssl http2;                                                                   
        server_name MY_DOMAIN_HERE;                                                            

        ssl_certificate /etc/letsencrypt/live/MY_DOMAIN_HERE/fullchain.pem;                    
        ssl_certificate_key /etc/letsencrypt/live/MY_DOMAIN_HERE/privkey.pem;                  

        root /var/www/;                                                                              

        location /nextcloud/ {
         #   rewrite ^/nextcloud/(.*)$ $1 break;                                                     

                proxy_set_header X-Forwarded-For $remote_addr;                                       
                proxy_pass http://127.0.0.1:5141/;                                                   

                proxy_set_header Host MY_DOMAIN_HERE;                                          
        }

        location /noVNC/ {
                proxy_pass http://127.0.0.1:5142;                                                    
        }

        location /shell/ {
                proxy_pass http://127.0.0.1:4200;                                                    
        }


        location /files/ {
                return 301 /;                                                                        
                proxy_pass http://127.0.0.1:5143;                                                    
        }

        #location / {                                                                                
        #       proxy_pass http://127.0.0.1:5143;                                                    
        #}                                                                                           
}