Learn Swedish: Vi hörs senare! - We will talk later!

You are right that the base verb höra means "to hear". But the reciprocal form hörs literally means "to hear each other". In Swedish, this is the standard and natural way to say "we will talk" or "we will be in touch", especially for phone calls or texting.

Is the -s at the end of hörs making it a passive verb?

Not here! While the -s ending is often used to make verbs passive, here it is used to create a reciprocal verb. This means the action goes both ways: we hear each other. Other common examples you will see are ses (see each other) and träffas (meet each other).

Why is the present tense hörs used instead of a future tense like ska höras?

Swedish frequently uses the present tense to talk about the future as long as the context makes it clear. Here, the word senare (later) shows that the action has not happened yet, so you do not need a future helper verb like ska.

Is senare related to the word sen (late)?

Yes, senare is exactly how you say "later". It is the comparative form of the word sen (late).