Also in this 919, not only are turbos(turbines?) being used to compress more mixture into the car, but also directly to drive a generator. So there must be unused energy in the exhaust (isn't this why highly tuned drag cars have fire literally coming out of the exhaust?)
The energy recovered by a turbo is not entirely a free lunch. A turbocharger gets some of its energy "for free" and some of it not so free.
To the extent that the turbo is acting like a turbine (extracting energy by dropping temperature and pressure across the turbine blades), you are recovering that energy "for free".
However, to the extent that the turbo appears as additional back-pressure to the engine as each cylinder's exhaust valve opens, that portion of the energy is not had "for free".
Theory tells us that the amount of energy left to be extracted from exhaust gas is proportional to how much hotter it is than the ambient air temperature (even 1 degree means there is energy left in the exhaust).
However, every practical Carnot-cycle heat engine (steam, piston ICE, gas turbine, etc) ever manufactured leaves some energy on the table.
Practically, the process of extracting additional energy from the exhaust means running it through another "stage" of the engine (e.g. an additional ring of turbine blades in a turbine or an additional cylinder in a compound steam engine).
But here's the rub: Have you ever noticed that in every turbine of every jet engine, each successive ring of blades is physically larger in diameter than the one before it? And in a compound steam engine, each additional cylinder is larger than the one before it?
This is because the process of extracting energy from the exhaust requires allowing the gas to expand, requiring that the next stage be even larger in order to extract additional energy.
So in any practical engine, you reach a point of compromise where you decide that adding another stage is either too expensive, adds too much weight (or perhaps adds just enough friction as as to be counter-productive!) that it isn't worth it to chase after that last few percent of heat energy left in the exhaust.
Edit: This is also why you see the advent of combined cycle power plants. At a certain point, whatever heat is left in the exhaust is more practically used to provide heated water, than to try and convert it to kinetic (and then electrical) energy.
Also, just to add to your comment, another form of energy that you get for free from a turbo application is harnessing the sound pulses from the exhaust. You wouldn't think it's much but all you have to do is compare the F1 V10 vs the V6T https://www.youtube.com/watch?v=jS4Dh_EAfJI
powered by exhaust gasses, which is known as a "turbocharger"
At the most basic level thermodynamic machines are about max delta T.
A turbocharger doesn't create energy. It's function is to increase the volumetric efficiency of the IC engine. How? By allowing more fuel to be pumped into the same size engine. It really is that simple.
Ideal combustion of gasoline occurs at the stoichiometric ratio, which is 14.7 to 1 (roughly, lots of variables). Let's call it 15 to 1.
That means you need 15 grams of air [1] for every gram of gasoline you pump into the engine.
How do you generate more power?
You burn more fuel per unit time.
How do you burn more fuel per unit time?
You build a bigger engine or you figure out how to pump more fuel into the same engine.
What is the limiting factor in being able to pump more fuel into the same engine?
You can't get enough air into a naturally aspirated engine in order to maintain the roughly 15 to 1 ratio. If you can't get more air into the engine you can't pump more fuel into it. Period.
How do we get more air into the engine?
You build a compressor. The compressor can be powered directly from the engine's drive shaft, in which case it is called a "supercharger", or it can be powered by exhaust gasses, which is known as a "turbocharger".
A turbocharger doesn't turn a piston engine into a turbine. It's function is to forcefully shove more air into the combustion chamber in order to allow a proportionally greater amount of fuel to be burned per unit time, thereby producing more power per engine unit volume, often termed "volumetric efficiency".
A greater volumetric efficiency does NOT mean an engine is more efficient. This is often a mistake made by consumers when buying cars. A turbocharger allows you to make more power with the same engine. It does not make it more efficient other than recovering some of the energy lost through exhaust.
Having owned several turbocharged and twin turbocharged fire-breathing monsters over the years I can tell you fuel efficiency was never better than non-turbo cars and, in my case at least, never a criteria for buying them. The latter said with a big fucking grin. And I have the speeding ticket collection to prove it.
[1] Actually, I think it's oxygen, not air. I don't remember, it's been decades since I studied this. I'll just call it "air" as a generalized yet possibly inaccurate term.