The house elves do. It says in book 4.
Selpek is not mentioned in Harry Potter nor does it exist in the real world.
Hogwarts students get breakfast, lunch and dinner everyday. There is a wide selection of food available in the Great Hall. In the fourth book we learn this is prepared by house elves.
Harry Potter. Your arguments are invalid. -Ting Ching
If you mean at Hogwarts, then it comes from the kitchens. The trio first discover the kitchens in book four, Harry Potter and the Goblet of Fire, later in which Hermione makes the Society for the Promotion of Elvish Welfare (S. P. E. W.)
Give him owl-food.
No, there is not.
Harry Potter. Your arguments are invalid. -Ting Ching
He ate everything except for the humbugs
The food at Hogwarts is made by house elves. There are four replica tables in the kitchens which are directly below the great hall. The food is loaded onto the tables and then appears upstairs.
It waits for small animals to crawl into the plumbing. That is why when Harry, Professor Lockheart, and Ron went down they stepped on small animal bodys.
Please note that Harry Potter is a fictional story.Using their magic, they can do pretty much anything... There are a few restrictions, mentioned in the books. For example, they usually can't raise the dead, and make them live again, nor can they produce food out of nothing.
As far as I can recall, Ron Weasley makes no explicit purchases during the course of Philosopher's Stone. He notably does not purchase anything from the food trolley on the Hogwarts Express, as he has no pocket money.