12V isn't a 'standard' USB-PD voltage, so pretty much none of them. Tons of phones charge at 9V, though. And some use 15V.
The regulators/buck converters you speak of are inside the phones these days. They want a higher voltage even if the battery itself is 3-4.2V, so losses from cabling are lower and you don't need a special 5A cable to handle charging at the fastest rate.
Most phones charge at 5v. Modern USB-C chargers can charge between 5v and 20v based on configuration.
Buck converts and regulators are cheap and small these days.