Yankee just means American (as in USAian)

Before the US civil war it was already a term for all Americans. Once the confederacy declared “independence” they wanted to drop all ties to the US government and its citizens. Thus, Yankee became a (an attempt anyway) derogatory term for those who remained US citizens who at the time were basically all located above the Mason-Dixon line. And it’s just sort of stuck since then. Those outside the US context just use it for every American. The most famous usage is probably the Vietnamese anti-American propaganda saying like “Go home, Yankee! Your government doesn’t care about you!” and stuff like that

On a personal note or whatever, half my family is from the deep south, so I also grew up understanding it as a sort of derogatory term used for those from the northern states. It’s pretty fucking weird to realize eventually that your family members are using an “insult” from the pro-slavery rebel government towards you (not me but my dad. So, also kind of at me…). Not to credit them too much though because I doubt they know really what they’re saying in that respect and they do it as a joking thing mostly from what I saw. Of course sentiments of “war of northern aggression” and “MLK was a troublemaker” (heavily censoring that second one…) are still alive even among people that most outside observers probably wouldn’t clock as super racist upon casual observation. When you’re their nephew or grandson though, well, I can only confirm all the assumptions people have towards white southerners. (No, not literally everyone, obviously, before someone comes yelling at me. But it’s a big percentage that are seemingly fine with everything from casual slur usage to aggressively wishing death on people. Obama being in office in particular sent my uncle down a pretty wild path)