When parameters.xml is used it's "baked" into
resulting Web Deploy package and then you can specify
parameters during deployment. So, you can use the same immutable
package and deploy it over and over to multiple environments.
I'm not very familiar with appSettings.json - do
they follow the same approach or it's build-time thing?
Then these 2 files are merged, with properties in the appveyor
file overwriting any matching properties in the actual settings
file. The way Asp.Net Core merges config you could just overwrite a
small part of the config file like this: