I don't understand when or why the "t" is added?

Bonjour Jenny !

In the case of inverted questions, you add the "t" for pronunciation reasons, whenever there are two vowel sounds clashing.
This is explained in the related lesson:
https://progress.lawlessfrench.com/revision/grammar/forming-questions-by-inverting-in-the-present-tense-il-elle-on-forms

However, thanks to you, I've now updated this lesson, by adding links to other lessons, as well as adding this explanation to it :)

I hope that's helpful!
Merci et à bientôt !