Learn French: Tu dois télécharger l'application avant de commander le repas. - You must download the application before ordering the meal.

In French, the phrase avant de (before) must always be followed by an infinitive verb. Even though English uses the -ing form ('before ordering'), French translates this literally as 'before to order' (avant de commander).

Could I use avant que instead of avant de here?

It wouldn't sound natural. You use avant de + infinitive when the same person is doing both actions (here, tu is downloading and ordering). You only use avant que when there is a new subject for the second action, like in 'You must download the app before HE orders the meal'.

Why is the verb télécharger also in the infinitive?

It follows dois, which is the conjugated form of the verb devoir (must / to have to). When a conjugated verb is directly followed by another verb to express a single idea, the second verb stays in its infinitive form.

Is it common to shorten l'application in spoken French, just like we say 'the app' in English?

Yes! In everyday casual conversation, French speakers frequently shorten it to l'appli or sometimes just l'app.