Differences between proxy_pass in two modules in nginx

server {
    listen 127.0.0.1:12345;
    proxy_pass 127.0.0.1:8080;
}
 
server {
    listen 12345;
    proxy_connect_timeout 1s;
    proxy_timeout 1m;
    proxy_pass example.com:12345;
}
 
server {
    listen 53 udp;
    proxy_responses 1;
    proxy_timeout 20s;
    proxy_pass dns.example.com:53;
}
 
server {
    listen [::1]:12345;
    proxy_pass unix:/tmp/stream.socket;

server {
    listen 80;
    server_name www.test.com;
 
    # Scenario A
    # Visit http://www.test.com/testa/aaaa
    # The backend request_uri is: /testa/aaaa
    location ^~ /testa/ {
        proxy_pass http://127.0.0.1:8801;
    }
    
    # Scenario B
    # Visit http://www.test.com/testb/bbbb
    # The backend request_uri is: /bbbb
    location ^~ /testb/ {
        proxy_pass http://127.0.0.1:8801/;
    }
 
    # Scenario C
    # The following location is correct ~ /testc {
        proxy_pass http://127.0.0.1:8801;
    }
 
    # Scenario D
    # The following location is wrong#
    # When running nginx -t, the following error will be reported: 
    #
    # nginx: [emerg] "proxy_pass" cannot have URI part in location given by regular 
    # expression, or inside named location, or inside "if" statement, or inside 
    # "limit_except" block in /opt/app/nginx/conf/vhost/test.conf:17
    # 
    # When location is a regular expression, proxy_pass cannot contain a URI part. This example contains "/"
    location ~ /testd {
        proxy_pass http://127.0.0.1:8801/; # Remember, when location is a regular expression, you cannot write it like this! ! !
    }
 
    # Scenario E
    # Visit http://www.test.com/ccc/bbbb
    # The backend request_uri is: /aaa/ccc/bbbb
    location /ccc/ {
        proxy_pass http://127.0.0.1:8801/aaa$request_uri;
    }
 
    # Scenario F
    # Visit http://www.test.com/namea/ddd
    # The backend request_uri is: /yongfu?namea=ddd
    location /namea/ {
        rewrite /namea/([^/]+) /yongfu?namea=$1 break;
        proxy_pass http://127.0.0.1:8801;
    }
 
    # Scenario G
    # Visit http://www.test.com/nameb/eee
    # The backend request_uri is: /yongfu?nameb=eee
    location /nameb/ {
        rewrite /nameb/([^/]+) /yongfu?nameb=$1 break;
        proxy_pass http://127.0.0.1:8801/;
    }
 
    access_log /data/logs/www/www.test.com.log;
}
 
server {
    listen 8801;
    server_name www.test.com;
    
    root /data/www/test;
    index index.php index.html;
 
    rewrite ^(.*)$ /test.php?u=$1 last;
 
    location ~ \.php$ {
        try_files $uri =404;
        fastcgi_pass unix:/tmp/php-cgi.sock;
        fastcgi_index index.php;
        include fastcgi.conf;
    }
 
    access_log /data/logs/www/www.test.com.8801.log;
}