This section of the Solidity course from FreeCodeCamp explains how to add the Mock contract to a test folder inside your contracts folder.
Create test folder in contracts folder
Create a new ...
A few things to double check to make sure its working correctly:
Run brownie compile to ensure most recent contract is compiled
Ensure environment variable in .env file is named ETHERSCAN_TOKEN
To add to the top upvoted answer, you can view all of the supported networks by typing brownie networks list.
The output you see comes from the network-config.yaml file located at ~/USERNAME/.brownie/...