How to query collections group in FlutterFlow, using custom codes

Hello! I am developing an app using Flutterflow and Firestore. What I'm trying to do is use multiple collections in Firestore, get the topping data for a particular menu and list it on the screen in FlutterFlow. However, I don't know how to write a custom code for this at the moment.

I'm trying to write a custom code after knowing that I can't do multiple queries at the same time in flutterflow, but I'm curious how I can access and write the exact page I want.

Only the main.dart file is checked in this capture, so I wonder if there is any conflict if I create a new file with the exact page name. What I'm trying to do is just making two queries on the page that I've already made with flutterflow, so I wonder if I do need to create a whole new page. ;)

It may be a basic question because I first encountered the flutter two weeks ago and I'm still lacking in English and coding experience, but I'm asking because there's no place around me to ask for help.

Thank you.

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