Yes, I am aware of the royal we but I think it's very limited in usage.
However, I didn't know that "they" can be used as singular. I think "he" should be used generally but I suppose if one is overly concerned about political correctness, he (yep I am doing it!) may use "they". Thanks for that!
There's a mod here on Duolingo who is a transgenre or doesn't want to have a genre, he/she wants to be called "they" as he doesn't want to make choice between "she" or "he".
"They" is used when someone doesn't know if it's a female or a male. But it's rather advanced English, and a rare case. Anyway, good to know
but for basic learners like in this course, it's better to stick to "they" = plural,
I think, and forget the "they" as "undetermined singular person". It could be way too confusing.