I'm going to follow on a bit from what jawilson said. The idea is this - you can measure blood oxygenation by sticking your head in a big magnet that makes atoms spin really fast and measuring the radio waves that come off. This is imprecise, but reasonably repeatable.

So if I show you a picture of a cat, and you like cats, then a bit of your brain might start using more oxygen because you're thinking about cute furry things, and if I show you a picture of a car, and you like cars, a different bit of your brain lights up showing more oxygen use because you're thinking about fast shiny things.

But really we've only got the barest idea of what bits of the brain do what, and maybe it's a bit of brain that goes "hey I'm happy" that lights up in both cases because you like both cats and cars.

We can kind of see bits we think are associated with muscle movement coming to life if I show you a picture of a bike, and you like cycling, and if I show you a really cool mountain track you imagine belting down it flat out. That lights up differently if I show you something else.

However, we do not really know except in very broad terms what bits of the brain actually do what. We can't "see thoughts", we just know that some bits of brain seem to use more oxygen than others, and from that we guess "this bit of brain is for thinking about sitting in a nice cafe with a cup of coffee and a newspaper" versus "this bit of brain is for being frightened of lions".

At least when phrenology was a thing, the ceramic heads with lines painted on were inexpensive and didn't require three-phase power and huge barrels of liquid helium.