What Ghasts even are? I honestly don't know what to point out
They're ghostly I mean, what else am I supposed to think by seeing a pale floating creature called ghast? but not undead mobs since they don't take extra damage from healing potions/arrows nor they take extra damage from weapons enchanted with Smite
They're fire+lava proof along with shooting fireballs but still can be burned by taking damage from swords enchanted with Fire Aspect and arrows from a enchanted bow with Flame
They're one of the few mobs with self-regeneration properties, the other ones being horses, Withers and Allays (the first one being arguable if it's merely for gameplay design) but it's not something inherent on them — instead it's on their tears — since they can't regenerate themselves
They also drop powder and... actually, wtf is even that thing? The more I look at it, the more it seems contradictory
The Mobestiary (which finds itself in this weird spot of not being canon while also being the only place where it explains a bit of things) seems to claim it's a mechanical creature but even this explanation doesn't seem to explain any of it
Also, the "Uneasy Alliance" (the name by itself suggests something I'm still unsure about) says we should rescue a Ghast from the Nether to bring it to the Overworld and kill it. I don't even know where to start, let alone concluding