Why CI4 Cannot Run in a Subdirectory?

Hi.

I've read somewhere that CI4 is not supposed to be run in a subdirectory (like Laravel?). What is the reason for this behavior?

In my previous experience, when I created a website for a client using CI3 or Wordpress, some clients usually want the staging version is uploaded to a subdomain (like example.com/staging/). What is the solution to this?

Thanks.

Perhaps you misread or the source was faulty. There is no "must be this way" file structure for CI4. This documentation describes how to relocate the application directory. Right below that is explained how to manage multiple apps which is what you describe as a staging version.