autocomplete.json is a plain, author-curated file of suggestion lists - the app only ever reads it at fill-out time, never writes to it. It's not a history of what people have typed; edit it via the Autocomplete Editor, or by hand:

{
  "make": ["Ford", "Chevrolet", "Toyota", "Honda"],
  "trim_level": ["Base", "Sport", "Premium"]
}

Each top-level key is a plain list of suggested strings. Wire a key to a field by setting that field's Autocomplete Key in the Form Editor to match - multiple fields can share the same key, and a field with no key set (or one that matches nothing here) just shows no suggestions, not an error.

Because it lives inside the project folder, deleting a project deletes it too, and copying/exporting the folder (including as a .yatate file - see Quick Start) brings it along.