Take a look at this, I would say, mess.
plant
A place where an industrial or manufacturing process takes place.
(from here)
mill
A building fitted with machinery for a manufacturing process.
(from here)
factory
A building or group of buildings where goods are manufactured or assembled chiefly by machine.
(from here)
Isn't it the same thing described by different words? I'm sure that there are some differences, even if they are not delineated in Lexico
Best Answer
It is indeed frustrating to find distinct words with dictionary entries that overlap to such a degree.
To get at the differences you will need to dig a little deeper, and maybe research the etymology. For example one attempt to differentiate plant and factory
I can think of two distinct situations
Going back to some origins:
Mill has a quite specific root in the grinding, such as wheat into flour, hence windwills. The term broadened to other forceful industrial processes. When I was a boy in Yorkshire my town had buildings where wool was processed into cloth, we called those Woollen Mills, and Lancashire had its Cotton Mills. Hence the stereotypical:
I would say that "mill" implies processing some basic ingredient into something that itself is used by another industrial process. The original meaning comes from grinding, mill-stones grind grain to flour.
A factory has connotations of manufacturing, creating complex objects.
A plant is not a word I remember being used in this way (as opposed to a living, growing, organsim) as a boy in the UK, but is certainly common now. I view plant as something larger than a single factory, a huge car manufacturing plant for example. I think plant can describe places where things are processed rather than manufactured, for example a water purification plant.
So to name something a mill we have relatively clear concept. A plant seems a broader term than factory. For things such as a water-processing plant, it's clear that plant is a better term. But why are the Nissan Assembly Plants plants rather than factories? I can think of no reason other than scale.
My default would be to use the word factory. I'd be confident of being understood.