Switching between different templating options in the same doc

The docs include several references to 11ty.js files, but I can't find any part in the docs that actually document them for beginners. Some questions a beginner might have include:

• What is an 11ty.js file for?
• Are 11ty.js files meant to be generated, copy-and-pasted from somewhere, or created from scratch?
• Do 11ty.js files undergo transformation, or are they just "regular JS files?"
• Are its contents automatically picked up, or does the file need to be referenced or added to the project in some way?
• Is the filename significant?
• Do I need an 11ty.js file?
• What must be included in at 11ty.js file in order for it to work? Is there a boilerplate?

My particular scenario: I was looking for a way to add custom filters, and the docs recommended that I add a snippet to my 11ty.js file. At this point, I realized I didn't have an 11ty.js file in my project and didn't know what that was. When I wanted to learn more, I wasn't able to easily find an explanation by browsing the docs.

Not sure if this is a case of missing documentation or just documentation being located in a non-intuitive spot. (I still haven't found it!)

Thanks!

Search the documentation for existing references to 11ty.js and the custom filters guidance, then trace where templating options are explained. Done means adding or locating beginner-friendly guidance that answers what 11ty.js files are for, how they are created and discovered, whether they are required, and what they must contain.