I've noticed that an application has a release with a comment "Auto-generated by a phone update". What does that mean and when does it happen?
As of CommCare version 2.34, users have the ability to update to the "latest saved state" of their app, without having to create a build first. (This is intended to streamline the process of iterative testing/debugging of a CommCare app). When CommCare HQ receives an update request of this nature, it simultaneously returns the information needed for an update to the user and kicks off a build in the background, so that the user can come back to the app version they updated to later. This is the build that you are seeing above.
I would also encourage you to read this page, which has full details on the different update target options in CommCare and how to set them.