The area of effect of dispel magic is already describe in the quote you used there: “one creature, object, or magical effect…”.

The point of confusion appears to be the idea that an AoE magical effect “must” need an AoE dispel magic to be properly targetted. But that's not the case: AoE magical effects are still singular magical effects and can be dispelled by a single-target dispel magic.

(Dispel magic would only need an “area” target description if dispel magic could affect multiple creatures, objects, and magical effects in a single casting, but it can't, so it doesn't.)

So if darkness was cast on a spot, it is an independent magical effect which you can target and dispel magic will affect the darkness spell itself. If darkness was cast on a creature, then you could target your dispel magic on the creature or the visible magical effect, but in practice you would always target the magical effect due to the difficulty of even knowing whether there’s a creature there to validly target.

Cast Dispel on the Target of the Magic Jar

The target of magic jar is always in the jar or possessing a body. Wherever that is should be the target for dispel magic.

In the description of the spell, the target of the spell is referred to as "you". Wherever that "you" is, is where the target of magic jar is, and should be the target of dispel magic.

Initially in the Container

The "you" is in the container.

Your body falls into a catatonic state as your soul leaves it and enters the container you used for the spell's material component... The only action you can take is to project your soul up to 100 feet out of the container, either returning to your living body (and ending the spell) or attempting to possess a humanoids body.

Possessing a Host Body

The "you" is in the host body.

While possessing a body, you can use your action to return from the host body to the container if it is within 100 feet of you, returning the host creature's soul to its body