Your joiner will most likely be in charge of fitting out the kitchen, internal doors, skirting boards/architraves and staircase if you go for a timber one or plan to cover your concrete staircase with a finish. Bathroom units are often bought off the shelf but can involve the input of a joiner; likewise with fitted wardrobes and bedside units.
The joiner will mostly be involved in the second fix, i.e. come to site after the plastering has been done, but they may have to set up some structural supports before this, e.g. in the case of staircases for example. Stud walls, door frames and other structural items will be going up at this stage also, work that is usually done by the builder/main contractor but if direct labour it will be a job for the joiner/carpenter.
The most common route self-builders take for fitting doors is to get the joiner to hang them individually; the doors would usually be bought from a manufacturer. A solid oak finish is popular, and more expensive, but if the door is to be painted pine (red deal) is more common and cost effective.
Prehung doors are available, which come with the frame, but tend to compare price wise when labour and parts are all included. Crittall style internal doors are gaining in popularity and traditionally use steel to frame rectangular panes of glass; a cheaper option is to use timber instead of steal. These are usually manufactured by a specialist company.
Some joiners will put together windows for you; this is mostly relevant to renovation projects that require non-standard replacements. Regardless of the type of joinery work that is getting done, you will need to get three quotes to compare and have a clear list of what materials you want to use, in return you can ask for a breakdown of materials and labour costs but oftentimes they will give you a lump price quotation.