Rooms in the various residences are permanent and are decorated depending on the owner. For example, at Windsor, the Queen is the boss and so she'll have the private rooms decorated as she sees fit but then the members of the RF will add their personal touches. At Windsor, almost all of the Queen's children have rooms and the same goes for Buckingham Palace. Princess Anne has rooms at St James's too and Princess Alexandra has rooms there also.
As for eating, they ring the bell, put in their order and it's sent up. Generally, they wouldn't be guests of the Queen because the rooms are considered to be theirs and they are private. They get serviced by the Household staff. As far as I know, Beatrice and Eugenie, Zara and Peter don't have rooms in any Royal Residence apart from (for B and E) Sunninghill and (for Z and P) at Gatcombe.
At private residences, the Royal Family can pretty much do as they like. For example, Highgrove belongs to Prince Charles and he can do as he wants. There's a tale of him going down to make himself a sandwich and he saw several chickens in the fridge. When he asked why there were so many, his cook said, "Well Sir, you do like your vol-au-vents". In Buckingham Palace, Charles wouldn't do that because it's very much his mother's house.
It's a very complicated subject and I'd recommend you get "At Home with the Queen" by Brian Hoey which gives a detailed account of every aspect of Royal life such as where the Royals sleep, what they eat and when etc etc.