In some cases, they are beliefs people hold and don't even realize they hold.
My ex was career military and we arrived at a new duty station and he became fast friends with a coworker. He talked all the time time about his coworker "John" this and "John" that (John is not his real name). He rather got on my nerves with how much he blathered on about John.
In all those months, he never once mentioned that John was black. It wasn't anything that made his radar at all as worthy of noting.
When I met John and his family, I was very surprised that he (and his family) was black. I had assumed he was white. With seeing him, I realized in an instant that I had made this assumption because I grew up in the Deep South and if you didn't mention skin color, the signal there was that they had to be white. If they weren't white, you should give other white people the head's up.
I realized in that instant that this was a racist assumption and I wasn't as immune to the racism around me as I had thought I had been. I still had been inculcated with practices I was oblivious to as being a problem in that regard.
The surprise showed on my face and I was not able to figure out how to explain that I didn't care that he was black, I was just shocked and appalled to realize that I had made this assumption and was having a come-to-jesus moment with myself. It made for a very awkward meeting.
After that, I tried to just let my kids model race stuff from their father and did my best to butt out. It's an uncomfortable incident that I thought of quite often for some years afterwards.
In part because of that incident, I can be pretty thick skinned about a lot of low level, run of the mill sexism because I'm aware that a lot of people are doing pretty much that same thing without realizing it. In most cases, it is easier to combat if I don't try to point fingers, make them feel guilty, publicly embarras them, etc.
I do sometimes make pointed remarks to try to educate people, but I spend a lot of time trying to simply be the change I would like to see and letting other people react to that as they see fit.
Thank you for engaging me. I did read your piece (and the piece it links to at the start as background).