Not to mansplain mansplaining but the egregious ones are men explaining things to women when the women are the experts. So some dude who has never so much as written a page, trying to explain something to an author with several books out. Or women who have Ph.D.'s in a topic and some layperson decides he wants to explain something about her field to her. It really does get ridiculous
That said, to at least some extent some of this is just men being dumb AF. So you have NBA players making millions, studying film, been in the league 10 years, all stars, whatever, and some 20-year-old on a message board says "Why don't you just do x! You're trash". And in the same vein, a whole host of men SWEAR they would "destroy" some role player on an NBA team, when I hope by now we've all seen the Scalabrine videos or other videos and know that no, you would absolutely get wiped off the face of the earth by an NBA player, you keyboard warrior
But yeah, it's tied into that irrational sense of confidence a lot of guys get when trying to act like they know something about something, when in reality half of it is pulled out of their *** or it's wildly oversimplifying things to the point its clear you don't have a good grasp (ie not just dumbing it down on purpose) and you should just say you don't know or make it clear you don't rather than trying to pipe up.
Now THAT said, the origins of that may be from years, at least from my experience, of the culture of manhood believing and training you that speaking up and saying anything is better than just sitting there.
But if it makes you feel some kind of way, just view it the same way you view American society and racism. You know we live in a racist society, so regardless of who you are, you're going to get socialized to do or think racist things, and you need to constantly self-correct. So if you mansplain something and get called out on it, okay, try to be better, just like if you say something that's ableist, sexist, transphobic etc. Nobody's perfect and we are hopefully all trying to get better. View it as state vs trait and keep it pushin.