Whilst the industrial revolution started with mechanical power; the harnessing of steam essentially. The modern world owes everything to the mastery of the electron and the photon, and more generally the exquisite control of electromagnetism.
The manipulation of electrons enables the iPhone; touted as the possible pinnacle of achievement; basically every industrial process on the planet is monitored and controlled by electromagnetic means.
Our true highest achievement is the ongoing development of the control of electromagnetic phenomena; we bend electrons and photons to our will, manipulate matter on the smallest scales using these forces, measure the bending of the fabric of reality, and probe the secrets of the universe. The iPhone is but one small aspect of this continuum of knowledge.
Whilst this is true; your body does have some pretty neat tricks to maintain homeostasis; it can shift the energy budget around quite a bit to where it is needed.
Your body will down regulate some systems to try to keep your total energy balance within what is "normal" for each person.
Digestion uses quite a bit of energy; this is why sometimes you feel sleepy after eating; your brain has been down regulated to enable digestion.
Another common example is when runners get into "the zone"; this is your brain prioritising the required processes and reducing the energy of other parts, putting you into a semi trance....this is so your body can maintain an energy balance.
It is also why we sometimes feel sick if exercising hard and then eat quickly afterward; your gut is not ready for that job.
High energy process that can be "switched off" or at least significantly reduced:
Brain processes (up to 25% of your energy budget)
Immune system (~20% when fighting infection)
Digestion (dependent on food 3[sugar] - 30[protein]% of food energy)
Just because you have done some exercise; doesn't mean you have used more total energy that day....it seems counter intuitive; but your body likely shifted energy from one thing (immune system, brain) to muscles, for the time your were exercising.
In saying that exercising is so good for other things; physical and mental health are enhanced by exercise, there are so many good things about exercise, just don't rely on it for weight loss.
As the old saying goes "you can't out run a bad diet"; you are correct, if over the long term you eat fewer calories than your body requires, you will see an effect. But your body is a tricksy beast, it will do all it can to prevent this; it is why dieting is so hard in an age of abundant food.
Kai Tangata by Alien Weaponry