As software that can be changed, someone controls bitcoin. I am interested to know who can change it.

In particular, I am interested in knowing if the 21 million can ever be changed? And if not, why?

I looked around Google for a while but didn't find anything that definitively answered those questions. I personally don't think the 21 million should ever be changed. And I would like the answer to be that it is impossible to change it, but I don't think that is the case.