1/ What are the most popular heat pump brands in Polish market ?
There is no leader in Polish market the most important parts of markets takes companies with experience in other areas type Viessmann, Danfoss.
2/ will the temperature go below -20C ? if that's true, how many days like this in a year ?
It is true. Last years we had 5-10 days with such a temperature, this year temperature goes even beloe -30.
3/ why are houses partly under ground level ?
Because in the ground level normally there is basemenet or some part of garage. People are acostumbred to cbuild such a buildings because it is quite convenient to have basement. You can store ther anything is not needen everyday but it will be needed sometimes like bicickles, or some food for winter. It is more question of culture than construction.