"Kateřina and Žofie are eating apples."
Translation:Kateřina a Žofie jedí jablka.
This is the spoken form of the standard language. It is not too formal for most occasions. Even when the other party of the conversation uses a dialect, you can speak using the standard.
Common Czech https://en.wikipedia.org/wiki/Czech_language#Common_Czech is shown in the last skill of this course. You would not guess how many complaints we receive from native Czechs for even showing the nonstandard stuff. Especially from those who use a different dialect.
The 3rd person plural verb endings have many dialectal variants.
"Jedí" is the standard literary variant.
It's not true that nobody uses it, many people do. And almost everyone uses it in writing, even if they prefer using a dialectal form (such as "jí", "jeděj", "jedijú", "jedijó", or "jijó") in speech.