I puzzled over this for a long time, and the simplest way I can lay it out is this:
The parts of a planetary gear system are ring (the outside), planets (the little whirly bits inside), and the sun (the central shaft). These are always engaged with each other. In our cars, the ICE is connected to the sun gear at a fixed ratio, the wheels and one motor are connected to the ring, also at a fixed ratio. The axes of the planetaries are connected by a yoke to the second motor.
Under electric drive, the ICE and second motor are unenergized, the planetaries spin freely. A load is placed on the either motor for regen. The real trick is that the second motor controls the "gear ratio" between the ICE and the ring and thus the wheels.
All of those modes are ratios of thrust or drag among the ICE and the two motors. For instance, to start the ICE, the motor on the planetaries spins against the ring to turn the ICE. Propulsion by ICE has the planetaries held still (the motor can do that), so that the power from the ICE transfers through to the ring. Varying the power fed to the planetaries can allow all three sources to contribute torque to the wheels.
Planetary gear systems are hard to think about, I think because thrust and drag can come from any of the components.