I think there are couple things to understand about bundles:
1. It is meant for the Mobile/Web SDKs to consume the built bundles. They are meant to be stored somewhere (CDN, Cloud Storage, etc). In the stackoverflow post, it seems like you are trying to serve bundles from a Cloud Function instance? In this case, you should manually construct your bundles using the Admin SDKs, instead of using the extension (https://firebase.google.com/docs/extensions/official/firestore-bundle-builder
2. That extension is a no-code solution for building and serving (caching via Firebase Hosting and Cloud Storage) bundles. What it does is to automatically create a Cloud Function instance, reading from a Firestore Document and using it as specifications of how bundles can be built.
If you are using that extension, in the specification Document, field `Documents` should be the document path that are supposed to be in the bundle, alternatively you can also specify a `Query` to use to build the bundle. `Params` are placeholders you put in your queries, they will need to be specified via http parameters when you request for the bundle.
Bundles are a complex feature, and requires some coordination between Server (or Extension), and Client SDKs. Please make sure you have a good understanding on how it works before you implement your feature.