What is a GitHub bot account?

A: A "bot account" is just another Github user account. You create it exactly as you would any other Github user account. In the Account Requirements section of github's Terms of Service there's a definition of a machine account: We do permit machine accounts:

What is a GitHub user account?

In other words, a "User account" can be for a "person" (the normal case), but it can also be used by an automated process (a "bot"). Which is precisely how Benbalter is using GitHub in the Change Agent project you cited above. Q: How do I get one? A: A "bot account" is just another Github user account.

What is a GitHub app?

GitHub Apps are first-class actors within GitHub. A GitHub App acts on its own behalf, taking actions via the API directly using its own identity, which means you don’t need to maintain a bot or service account as a separate user. GitHub Apps also have a higher API rate limiting threshold than requests from user accounts.