Learn Spanish (Spain): Mi hermana dice que la logística del envío por avión es compleja. - My sister says that the logistics of the shipment by plane are complex.

While you will often see en avión when talking about a person travelling 'by plane', por avión is specifically used to describe the method or medium used to send something (like a shipment, package, or letter). The preposition por here translates to 'via' or 'by means of'.

The English translation says 'the logistics... are', but the Spanish says la logística... es. Why is it singular in Spanish?

In Spanish, la logística is a singular feminine noun, so it takes the singular verb es and the singular adjective compleja. English words ending in '-ics' often look plural and take plural verbs, but their Spanish equivalents are almost always singular.

Where does the noun envío come from?

Envío is a masculine noun meaning 'shipment' or 'delivery'. It is related to the verb enviar, which means 'to send'. In this sentence, del envío combines de and el to mean 'of the shipment'.

Why is the adjective compleja feminine if the words right before it (envío and avión) are masculine?

The adjective compleja (complex) describes the main subject of the clause, which is la logística (feminine singular). The words del envío por avión just add extra information about the logistics, so they do not change the gender of the adjective at the end of the sentence.