How to reference data from subcollection without running another query

Hey all, FF n00b here. I'm in the process of setting up Firebase for a CMS in which items will be grouped together and the groups will be displayed to the user. Some groups will have 1 item and some will have more than 1. The group contains metadata fields like groupType and groupLabel.

I'm wrestling with two questions, the first is whether I should be using subcollections at all. I understand that I could alternatively just have a Groups collection and an Items collection and have a groupId field on the latter. Is there a performance or other benefit from making it a subcollection?

And secondly, how do I access the Item subcollection documents in FF after I've queried the Groups collection?

Thanks!