The problem with Advanced Highers is that not ever subject will run in a school. I think my school has 3 subjects running AH this year. Dd2's school is running about 5. We are in Glasgow and are lucky to have the Advanced Higher Hub at Glasgow Caledonian University. They offer AH in maths, chemistry, physics, English, business, history and modern studies. So Dd2 goes there for 2.5 hours after school on a Wednesday and Friday morning every week for physics (she is lucky that maths and chemistry are in school).
For other subjects pupils will go to a nearby school just for those lessons. And sometimes it just doesn't workout. Dd1 wanted to AH computing but nowhere near enough was running it.
It is also complicated by some universities counting them as additional qualifications and others ignoring them as double counting unless you are upgrading (B at AH =A at Higher etc). So some pupils who have an A at Higher in their degree subject are better off not doing it at AH so they can study a different subject at Higher which will be counted. This is most important for those who did not get top grades in Fifth year.