While working with iOS 14 and IAPs, I was looking for the Sandbox Account management section in iOS Settings. Older guides online reference “iTunes & App Stores”, but it is somewhere else now.
From what I’ve read online, it seems like the 1st step is to attempt an IAP on your iPhone running your development build (with Xcode). You’ll be prompted to sign in, so use your App Store Connect Sandbox Tester credentials.
Afterwards, I was able to find Sandbox at the following location:
* open Settings app
* scroll down & select ‘App Store’
* scroll down & find ‘SANDBOX ACCOUNT’
Here, you can manage or sign out of your Sandbox Apple ID.