I read that mummies were taken their inner organs except the heart. How do we know this was true rather than that the organs demanifested like they do in normal dead bodies except the skin that was offcourse kept iby the egyptioans intact?

We know they were removed because most were mummified and kept close to the corpse, in various time periods. Sometimes in jars (Canopic) sometimes in large chests divided into separate chambers, sometimes wrapped in linin and replaced it the body cavities.
Quite often, when a tomb is excavated, the Canopic jars are still there--it's the mummy that's missing!

That's true.
In pre-Dynastic times, people would bury the dead in the desert. The natural processes of the body in the sand would lead to mummification.
In fact, it is theorized that it was this naturally occurring process that may have given the ancient Egyptians the idea that the body must be preserved.
Ironically, the very advancement to taking greater care of the body, burying it in elaborate graves, only hastened the decomposition. It was then that the ancients started to dry out the body, or mummify.