One day, I discovered Kim Jung Gi’s Youtube videos. How could he draw these realistic scenes from imagination? I looked up every interview with him I could find, and I learned that he had practiced drawing from memory.
I tried the drawing from memory exercise also, and the results blew my mind! Apart from the fact that I could now draw that thing from memory, I could draw OTHER things better also! It made me better at sensing when a line was wrong.
Being able to draw from observation is still the basis. You need to also be able to draw from observation first.