Your categories are will be saved in FireStore. FireStore is more advanced than Firebase Realtime Database.
You should already have created a Firebase Project. Automatically you get a FireStore project also.
But thanks to the included FireBase admin panel, you will manage your data through it.
If you have successfully followed the install procedure from you should have a menu similar to this one.
Step 1. Go inside the categories
Step 2. Adding your first category
When you go for the first time in your collections, a pop-up will appear that allows you to add your first collection.
Pressing on the ADD button will add the first collection.
Step 3. Managing your collections
You can add / delete / modify your collections. In the search bar, you can filter your collections.