How to learn trade in Germany if you can only work 3 days a week and cant live financially?

Hey, I am from 19 years old from Serbia and i plan to move to Germany. I’m a bit confused about how Ausbildung works in Germany, especially for trades like construction.

From what I understand, you work like 3 days a week and spend 2 days in school for 3 years learning trade. But the pay seems pretty low, i wouldnt be able to pay for basic rent, so I don’t really get how people manage to live like that. How Goverment think anyone without owning basement or having parents help learn trade?

Would appreciate some help, thank you.