How do high skilled service jobs fit into this picture?
The main key is having just some barrier to entry.
If you know how to repair motorcycles, you're going to be around more people that can afford motorcycles in a city. They'll be more expensive ones on average and people will be less likely to do the work themselves (time is worth more or they don't have a garage to do the work)
There's a shop in Chicago that only works on mopeds. They charge $75/hr on the workbench and that's for relatively cheap machines. Wait time without a rush fee in summer is on the order of weeks because they are the only non-independents in the game.
Due to the population, you can also run pretty niche services/shops. I know a place that only does battery sales and services. Phone, automotive, household gates, and so on.