What do you think "reality" is?

This I do not have a thoughtful opinion about, except to say, perhaps, that there is no one reality, there is only perception.

Are things that make NO sense to anyone aside from the "beholder" necessarily a sign of insanity?

My belief on this is no, not necessarily. But the further away from the mainstream beliefs about any given topic the more insane a person is going to appear to be to the collective. But who is doing the judging? And how can they really know they are any saner than the person with the "off the planet" beliefs?

How do people arrive at their opinions?

It would appear (through the oodles of research that's been done on early childhood cognition and the role of trauma in the development of and damage to the human system) that most of our opinions are created based on what we observe from our primary caregivers and those in positions of authority when we are under the age of 7. Add to this the traumatic experiences we have (that we don't manage to process/integrate/resolve at the time) and we get a human mind that has opinions that may or may not match much of what is believed by those around them but that make total sense to what they were fed and experienced in their life.

Most people will never really review what they believe to be true. Most will not revise their opinions and will carry their beliefs and opinions around with them like a medal until the day they die. Unless, like you and I appear to do, they choose to actively review them and are willing to revise them if they can see that what they once thought does no longer make sense.

I have noticed that there is a HUGE number of people who have been incredibly triggered during the last 2 1/2 years in particular. I have seen very clever people fall down rabbit holes and come up into online and offline spaces sprouting things I consider nonsense. It has helped me immensely to realise that those who have experienced significant trauma as a child have a mind that really and truly does see the world with a very specific lens that looks like a fairytale (or nightmare) landscape to me.

But what do I know really? I cannot be objective, for I, like them, am human. Still, my study of human beliefs, biology and behaviours continue because it's just so darn fascinating. Of course, the more I learn, the more I realise, the less I know ;)