How do I learn business? At home

I’m 21 and studying mechanical engineering but want to also start some businesses with my cousin in the future once we have some money saved and proper jobs. Hotels is really our main mission but how do we even go about that. He’s studying business accounting so he’ll have the knowledge but how could i understand basics and advanced stuff. Things like loans, and starting one out, understanding the process of say constructing a business after buying some land. How would write offs work. Etc..

It’s my first post on this sub but if there’s already posts about it please lmk. I’ve thought about doing harvards business courses and self studying, obviously wouldn’t get the degree but I just want the knowledge of it. And if anybody has started a small business that has grown how did you go about it?