Is it okay to use a 100-240V AC power adapter for charging my iPhone?

Yes, I do every day - most of the world uses 220 - 240 V.

See also Power adapters for iPhone

I haven’t seen any USB power adapter that isn’t. Every Apple OEM one would be. But what specifically?

That’s pretty standard. They all use switched mode power supplies that will convert to DC from a wide range of AC input voltage.