A description of each instance, including IDE-settable properties, is written into a hierarchical .yaml file. This file exists behind the scenes, and is used by the IDE to render the form in the Design view.
At run-time, Anvil reads this description to generate a base class (…Template) for the entire Form. You can see this base class referenced near the top of your Form’s Python code.
The base-class code is generated on-the-fly, so it is not accessible. The .yaml file is accessible via Git, but editing it is really a last resort, as it is extremely easy to screw it up that way.