I use Lightroom Classic og PhotoShop for editing. I have a Samsung monitor connected to my computer, not an expensive monitor. wonder how I can benefit from using ProPhoto RGB or Adobe RGB, if my monitor uses a smaller color space corresponding to approx. sRGB.
You are not using ProPhotoRGB to benefit from it on your monitor. There is no monitor in the world that can display ProPhotoRGB. There reason to use it is so you do not lose any colors when you edit the image. Colors you may not be able to see on your monitor perhaps, but that may contribute to a better quality print, for example.
It sounds like you are concerned about the quality of the colors you are seeing so a couple of suggestions:
Calibrate your monitor. This will show you the coverage you currently have.
Buy a good monitor. I use an ASUS ProArt PA249 as my main monitor. This is calibrate with a Spyder X. One interesting thing is that the changes between calbrated vs uncalibrated images is very little. I also have a cheap ACER secondary monitor which does benefit from calbration.