An ICE hybrid is a gas car with a little electric motor shoehorned inside.
A "plug in" hybrid as they are called is a full electric drivetrain, with a gas generator like you'd buy at Lowes stuck in the boot
.
It seems trivial, but the difference is massive. The former is super complicated, heavy, and expensive, as you need all the junk a gas car needs and the electric stuff to go with it.
The later is hilarously efficient. It takes the best part of electric cars, the dead simple drive train, and solves their achilles heel: the massive battery. You can get away with a dirt cheap 3 horsepower generator in such a setup and shrink the battery massively, whereas a ICE hybrid needs a huge car engine and (like I said) all the expensive junk that goes with it.
You don't see more of the later because:
Car manufacturers are geared to produce ICE cars, and reserve the electric drivetrain capacitry for profitable luxury vehicles first.
This is just speculation on my part, but a gas range extending generator "taints" a full electric car, making it unpalatable to people who think it ruins the image, eco friendliness or whatever, when it's actually better for the environment because the battery isn't so freaking big.