It's like the energy costs of those supercomputers used for modeling or training CNN models.

Yes. But you're not just a computer, so it's good to use the rest of your body. This isn't a cyberpunk dystopia yet.

Yes: https://www.frontiersin.org/journals/psychology/articles/10.3389/fpsyg.2018.01005/full

So apparently, a brain daydreaming or one fully immersed in a complex activity don't exhibit that much difference in energy usage, only consuming around 100 calories extra a day for mentally challenging activities.

Maybe, the brain can metabolize sugar or ketones. Kinda like a hybrid car that can run on gas or batteries.

https://www.frontiersin.org/journals/molecular-neuroscience/articles/10.3389/fnmol.2021.732120/full

Heck yeah it do. The brain is powered by glucose, and more brain activity will use much more of it. Jokingly, it's how I can tell a new engineer from one that is extremely experienced: A bright young engineer is usually skinny from problem solving all day. An older one is likely stuck in more meetings where brainpower is a liability and is probably on the heavier side.

Also, as an occasional eater of magic mushrooms, I keep packs of glucose in the house that are typically used for diabetic emergencies. Psilocybin pushes a brain into overdrive and causes my blood sugar to nosedive. (There are studies about possibly using psilocybin as a diabetic treatment to improve pancreatic function, btw..)

Yes. Look at how many calories professional chess players are burning by just sitting and playing chess. You will be shocked.

Have a refreshing SodaPop Zero and tell me all about it.

Teach me your ways, Master

Same. It took me years to figure out how to preempt it.