I think the song The Morning by The Weeknd is not primarily about prostitutes. There are lyrics that talk about prostitution but I think the context is about different ways of making money. I think the song is more about how fame and money influence your life, sometimes for the better but sometimes for the worse.
Personally, I do not like the song because I think it is degrading to women. The references to prostitution make it sound like women are objects to be played with and paid for if you have enough money to spend on them.
I do not believe that the song The Hills by The Weeknd was about Ariana Grande. It is clear that the twos artists like each other and collaborate well together but I do not think they are romantically involved.
The Weeknd has slept with many women so it could be about any one of them. If The Hills is about Ariana Grande, maybe there is someone else who has romantic feelings for her other than The Weeknd himself.