ss -tlnp:

State            Recv-Q            Send-Q                       Local Address:Port                        Peer Address:Port           Process                                              
LISTEN           0                 4096                               0.0.0.0:10000                            0.0.0.0:*               users:(("mongod",pid=10375,fd=11))                  
LISTEN           0                 4096                         127.0.0.53%lo:53                               0.0.0.0:*               users:(("systemd-resolve",pid=561,fd=13))           
LISTEN           0                 128                                0.0.0.0:22                               0.0.0.0:*               users:(("sshd",pid=630,fd=3))                       
LISTEN           0                 100                                0.0.0.0:25                               0.0.0.0:*               users:(("master",pid=1312,fd=13))                   
LISTEN           0                 511                              127.0.0.1:38011                            0.0.0.0:*               users:(("node",pid=11744,fd=18))                    
LISTEN           0                 100                                   [::]:25                                  [::]:*               users:(("master",pid=1312,fd=14))  

ps -aef | grep [m]ongo:

mongodb    10375       1  0 17:46 ?        00:00:15 /usr/bin/mongod --config /etc/mongodb.conf
root       11896   11809  0 19:27 ?        00:00:00 /root/.vscode-server/bin/b3e4e68a0bc097f0ae7907b217c1119af9e03435/node /root/.vscode-server/extensions/mongodb.mongodb-vscode-1.0.1/dist/languageServer.js --node-ipc --clientProcessId=11809

I myself did not specify any port number in the config.

Thanks for your help.