Is there a way to read configuration files from multiple applications in the IIS worker process with relative path specification?

For IIS+.NET Framework, use MapPath.

Specifies the relative path from the web root, not from the current folder.

string fullpath=Request.MapPath("~/config/aaa.config");

~/ will be replaced by the IIS root folder AA app in the top directory of the application.
If you have configured a virtual path in the IIS Management Console, you can view the real folder that corresponds to that virtual path.

Important: Security Considerations

IIS should be careful to avoid accidents where the configuration file is unexpectedly visible to the outside world.
I think the extension of the configuration file should be .config, not .xml.
IIS defaults to allow and deny HTTP access with extensions.
Even if you forget the per-folder deny setting, if the extension denies it, you can prevent the files from being viewed incorrectly from the outside.