Learn Italian: Anche se il deserto è arido, dobbiamo continuare. - Even though the desert is arid, we must continue.

Because it states a concrete, real fact. When anche se means "even though" to describe reality, it triggers the indicative mood. The desert actually is arid, so we use the present indicative è.

How would the meaning change if we used the subjunctive and said Anche se il deserto fosse arido?

Using the subjunctive changes anche se from "even though" to the hypothetical "even if." If you used the imperfect subjunctive fosse, it would mean "Even if the desert were arid," implying that it might not be, or it is just a hypothetical scenario.

Why is there no preposition between dobbiamo and continuare?

Dovere (dobbiamo) is a modal verb, similar to potere (can) and volere (want). Modal verbs attach directly to the infinitive of the following verb without needing linking prepositions like a or di.

Can arido be used metaphorically in Italian, or does it only describe physical dryness?

It can definitely be used metaphorically. While it literally means a physical lack of water, Italians frequently use arido to describe a person who lacks warmth, emotion, or empathy, or to describe a dry, uncreative situation.