I spent some time looking at this and it depends which hybrid, there is quite a bit of variation in drivetrains even within manufacturers. As sibling comment [1] describes the planetary gear arrangement in most Toyotas is quite simple compared to a normal automatic transmission, and the traction motor/generator replaces both the starter motor and alternator that a plain ICE engine would have. At the same time the hybrid system makes gentler use of the gas engine and brakes so even though they are mechanically near identical to the ICE model they will probably last a lot longer.