Azure App Service is PaaS. This means that we have very limited capabilities of changing configurations. applicationHost.config file is one of them. It holds all the necessary settings for your Web App but it is read-only.

Location of the applicationHost.config file

You can find your Web App's applicationHost.config file in the following location:

D:\Local\Config\applicationhost.config

OR

https://webappname.scm.azurewebsites.net/api/vfs/localsiteroot/config/applicationHost.config

It loads the same applicationHost.config from D:\Local\Config

It is read-only and you cannot directly edit it but you can use it to check certain settings and how they are configured.

Making changes to the applicationHost.config file

The file can be read-only but we have certain ways of making changes. It can be done with Xdt Transformation

Here are some examples:

https://github.com/projectkudu/kudu/wiki/Xdt-transform-samples

https://github.com/uglide/azure-content/blob/master/articles/app-service-web/web-sites-transform-extend.md

https://github.com/projectkudu/kudu/wiki/Azure-Site-Extensions#understanding-what-could-go-wrong-with-xdt-transforms

You can do a lot of the changes suggested for the Xdt examples through web.config so the use of Xdt transform must be a very specific