There are many ways to make a CVT at least in theory.

Commonly used though is a planetary gearset. A differential itself can be used, but I haven't seen that application in production yet.

You have a mechanical power path and a variator power path. The engine is generally coupled mechanically to the ground if a clutch isn't present. The "ratio" of the gearing depends on what the variator is doing.

Electric cars are using 2 stages of planetaries and clutches now in some applications for efficiency in different speed zones. Look up Voltec Gen 2 by GM. It has 4 or 5 different modes where as a simple planetary generally will have 2 or 3 modes.

This is Agco's CVT explanation video that shows what the variator path would be doing:

https://www.youtube.com/watch?v=dgtIKMAjvFI #mce_temp_url#