Firestore data model best practices

I tried to post a few times to get answers on some apps I am trying to recreate for educational purposes. Not a class project, this is just me learning for myself.

What is the best way to create a multiple nested document structure such as: a recipe app, a holiday card app, a baseball car collection app.

I am trying to learn the best way to structure data. I already know how to use the realtime Database. I would like to know the best practices way to structure Firestore Data.