In order to have the posibilities of adding complete elements, let's say add new events, with all fields inside, we will need to have the "Schema" definied so the admin panel to understand to how the new elements should look. When you define your schema, your will see a "+" button on top of each table, allowing you to insert new items.
The schema should be defined in "src/config/schema.json".
If you are just starting with your Firebase, on a fresh new project, and you have some small number of dataitems there, you can export your current data as json, and use it as your schema. You can modify the first element of each section with some general data.
If you are already running a firebase instance, and have big amount of data, best will be to define the schema on your own, folowing the patern from your database structure. . Or you can export your data, and remove element from your data, leaving only one per each point in your data structure.
Since v2.0.0 the ordering of the schema fields control the ordering of the data inside the admin panel.