It's not a conventional CVT. Instead, it's an Ecvt. They are very reliable!
There is no belt, chain or other mechanical way to change gear ratios. The Toyota/Ford hybrid system uses a engine/planetary gearset/motors and generators. The engine is always connected to the wheels. The planetary gearset is managed by motor and generator speed to any given RPM and road speed.
Look in the middle of the two motors in this pic for the planetary gearset.
It is difficult to explain the principle, but if you had a demonstrator in your hand, you'd see the principle immediately. A planetary gearset won't transmit any power unless (for example) the ring gear is held still. What if we could hook that ring gear to a motor/generator and change it's speed without losing energy? We could then provide an infinite number of "gears" to drive the wheels.
Here is a good video that shows how it works. Keep in mind the chains are for the demonstration: