Apologies, I might have missed the answer to this question, but given that definition of a creature, how does one rule on magic missile being cast to hit a creature which is secretly an illusion?
Multi-layered answer and I didn't scour the first 40 pages of posts. First, the original post regarding whether magic missiles affect mirror images has been adjudicated by Sage Advice, which Wizards holds as "official" rules clarifications.
Sage Advice says Magic Missile is unaffected by Mirror Image. It will hit the creature you're aiming for, even if you are seeing double. In short, by their nature, Mirror Images deflect things that require an attack roll, nothing else.
But, other illusions, yes
Magic Missiles can be aimed at an illusion, if it's a creature illusion. Let's start with
Minor Illusion, the cantrip, which can only create an object. The more powerful version
Major Image can generate a creature, and this distinction is important because each illusion creates something that can be targeted by some spells and not others.
Illusion spells "
deceive the mind." Jeremy Crawford once said in a podcast (non-canon) regarding "
invalid targeting" that he envisioned the spell creating a link between the caster and target. Generically, illusions aside, if you targeted a non-creature with
Magic Missile (e.g. a gargoyle statue), there would be no link, the spell couldn't work, and you'd lose the spell slot. This is consistent with the subsequent (optional) rule in Xanathar's Guide for Invalid Targets (adding that if the spell had a save, it'd appear the target saved so the caster doesn't get a heads up about the true nature of the target).
Notably, illusions are described as spells that "deceive the mind." The caster is fooled into believing there is a creature to target, and this deceit will be dispelled when there is physical interaction with the illusion. Once the missiles interact the ruse is up.
So, in summary. You can aim Magic Missile at anything you want. If it's a creature illusion revealed on physical interaction, your missiles will at least make contact and tell you it's not real. Mirror Image is distinguished because it only deflects things that require an attack roll, and that conversation has been exhausted enough I won't repeat it other than the conclusion that you can't aim Magic Missiles at Mirror Image duplicates.
As to the Golem question, a golem is a construct and counts as a creature. The Xanathar Guide rule on Invalid Targets would, however, make your spell invalid if you believed a simple statue were a golem. Depending on the spell, the slot would either be wasted, or the target would appear to save.