Apt v3 comes with some quality-of-life stuff that apt has been direly missing for years, and one of those is apt sources that make sense to a human.

So the new apt source files have local key location, components, suites, and Uris.

The sources from the before-time just had a one-liner with a uri and some components.

Edit:also the modernize command makes a backup of the source files it touches.