Bob Minchin wrote:
So how does the petrol engine in a hybrid vehicle start?

Does it initially drive on battery and engage the drive train somehow to
turn the engine and bump start it?

https://www.youtube.com/watch?v=ZmHpSyTsfm0

Essentially there's an ICE, two motors and a planetary gear system.
Depending on the field applied to the motors, they can either apply or
receive (regenerate) power. One of the modes is that a motor can turn over
the ICE and so start it. The design is actually simpler than a regular car
(ICE+gearbox+starter+alternator)

The control system is such that it's smooth through the power curve - it can
move power between the two motors, starting and stopping the ICE as needed,
depending on how much power is required and the state of charge of the
battery.

The Weber Auto YouTube channel above is great for explanation of how
transaxles and hybrids/EVs work.

Theo