A mill is known to be a place wherein you would need to do some grinding work. There are different things that you can grind, such as flour or rice. When you say factory, this is a place wherein there are different things that you can work on.
There are many things that are being done in a factory. There are different machines and tools that are present to ensure that the various tasks that need to be done in a factory will be completed.
There was a time when a mill used to refer to a factory, but this had changed over the past years, especially when it was apparent that there is already an obvious distinction between the two. Factories will usually need more people to work as compared to mills.