Lydia probably doesn't know as she's probably never lived in Scotland (and I don't blame her), but "aye" is still very commonly used in Scotland to mean "yes". My parents live there and I hear it all the time when I'm there visiting them. Your best bet is "yes". "Yeah" is also used, which is slightly more informal, but almost everybody says it, especially between friends.

In the United States, "yes" and "no" are the common, ordinary, basic words that we all use every day. "Aye" and "nay" are old-fashioned, unusual, and used only in few specific situations. The only place where I've heard it used is in a formal meeting where a "voice vote" is being taken. The custom is for the chair to say "All those in favor, say 'aye!'" And a lot of people say "Aye!" He then says "All those opposed say 'nay!'" And people say "Nay!" The chair looks, listens, and judges, and then announces "The ayes have it!" or "The nays have it!" In the U.S. Navy, and probably other English-speaking navies, I think it is customary to say "Aye, aye, sir!" in response to an order. It means more than "yes." It means "I acknowledge that this is an order, I understand what I'm supposed to do, and I will do it."

"Yes" is what you would say normally, any time you're wanting to affirm something. "Aye" is an outdated word from Middle English that would have been used by maybe a pirate ("Aye aye, Captain!") but never in normal life these days. Alternatively, sometimes when taking a vote, those who are voting "yes" or in favor of something might say "Aye." If they win, sometimes we say, "The Ayes have it!" meaning that those voting "yes" have beaten those who voted "no."

I always say "Aye!" and not "Yes". Never forget the Highland Clearances. So many families were involved including both sides of my own. I live in Australia and when it's appropriate I often hit up my friends with a Scottish accent and talk Gaelic at them from time to time lol. Aye and yes are hella interchangeable. ❤️ Have never been to Scotland, but it'll be my home one day guys!! 😂❤️

I echo Laurence regarding Scotland. "Aye" is also common in many parts of England, especially in the north.