Higher GPM shower heads usually cost more, so this certainly passes the “it’s expensive to be poor” test in that those who can’t afford a $75 shower head eventually pay for the difference and possibly then some.
Sometimes the restrictor looks like flat plastic cap with holes added.
Most of the time it’s an o-ring and the process looks like this: [0]
[0] https://www.tiktok.com/@hopopro/video/7261933672025132331
They look like little metal or plastic screens, usually. In the US, all shower fixtures must come with them installed by regulation.
For example, when you want to clean something with a garden hose you restrict the flow with your finger on the end to increase the pressure.