What means adding a backdoor in a web application for use with MSFVENOM and msfcli ??

Hello.

I would like to share this to know how to send a backdoor in a web application ??? Must we always look with a tool like DirBuster for example, all hidden files to find the direction or the file directory "upload.php" such that uploads malicious applications in the application of web fain exploit later with msfvenom and msfcli ???

And if this PHP web application in question does not have a upload.php file to upload backdoor ?

So how do to insert her backdoor or do you think that all web application still have pages like "upload.php" to upload backdoor ???

Thank you to answer me please.