Question about mechanics of Fireball and Firebolt.

So I was wondering, how exactly does fireball and firebolt deal damage?

It has two components - projectile and explosion. So my question is, what happens when it hits an enemy? Does it deal hit damage AND explosion damage to the target? Or hit damage to the target and explosion to surrounding targets?

Since we don't have target dummies it's really hard to test. But from what I observed, if I put Concentrated Effect support into firebolt, it reduces damage of the hit and increases damage of explosion. And it LOOKS like it deals more damage to single target as a result.
But I'm not 100% sure.

What do you guys think?