Footnotes are tricky to pin down because the general idea is "cite something that you yourself took from something else instead of creating it on your own." That's great and all, but considering a huge portion of our knowledge is learning from someone else, that's kinda hard to figure out. For example, let's look at this paragraph:

During the American Revolution in 1776, George Washington was Commander-in-Chief of the Continental Army. He faced funding difficulties and many times his soldiers were without pay or proper equipment. After being forced from Boston and New York, he won some victories against the British in New Jersey before culminating his victory at Saratoga and Yorktown with the help of the French. Later he became the first President of the United States and served two terms before retiring to a quiet life in the country.

I could literally put a footnote on every single sentence because I never personally did the research into any of that; I just got it from history class. And even if I did, I'd still have to cite all the letters and papers from the 1700s to prove my point. So usually another rule states, "Don't put a footnote on common knowledge." Except "common knowledge" is really vague. I knew all that shit off the top of my head, but knowing students and history I doubt most of them would be able to. So for that audience would I have to write that down?

Let's take another example: My professor complained that I didn't have a citation for 1000 AD as the year Iceland converted to Christianity. I countered practically anyone even remotely acquainted with Icelandic history would know that since it's an easy date; being a medievalist he should've known that. Because then what? Should I prove 1066 was the Battle of Hastings? 732 was the Battle of Tours? 800 was when Charlemagne became emperor? Yeah, none of those dates are "common knowledge," but considering this paper is between myself and my medieval history professor, I thought this would be a given.

To this day I still have no idea when I should add these because you have to make an assumption about your audience, which is a very tricky matter. I've seen papers go overboard and have more space dedicated to footnotes than actual paper. Or those that have too few and I can't check out the source. So I suspect no one really knows and it has to be by discretion, which leads you to having arguments with your professor over whether 1000 AD is a thing.