Measure your doors. Popular widths are (just under) widths of 600mm, 500mm, 400mm. Also available are 300mm, 800mm and strange odd ones (but not so widely). All sizes are ever so slightly smaller than the nominal size so they open and shut without banging or rubbing on each other.
Also measure the distance between the centre of the hinge, and the top and bottom of tge door. There are a couple of standards and it is much quicker to fit doors that match your cabinets. Your supplier will know about that. If you have any cabinets where the screw holes have been damaged by allowing them to get loose, they can be repaired with ease using cabinet hinge repair plates.
Heights are mostly full doors, and drawer line. Over the past 20 years or so the height has slightly altered, so the taller ones stick out slightly below the fronts of older cabinets, but you will not be able to see it unless you lie on the floor, and you will not notice it if you fit the whole kitchen so they all match.
The most durable cabinets are laminated with a hard plastic similar to worktops. You can get them patterned and with fake woodgrain, but IMO they are not quite convincing, and it can be hard to match up the patterns on adjacent units. Laminated doors are pretty certain to be flat slabs. They can easily be made to odd sizes, and fully edged all round with laminate, because they are cut from large factory-made sheets. Egger produce a very wide range of boards, which are used by many makers, and you can order samples.
The least durable doors are vinyl-wrapped chipboard or MDF. They are available in numerous moulded shapes. They are made by applying a thin layer of plastic film to the moulding with heat and pressure. They are cheap and easy to wipe clean, and are OK in a room that does not contain anything hot, damp, or steamy, such as a kettle, hob, sink, oven or dishwasher, when the vinyl will crack and peel. I have even seen them peeling in kitchen showrooms, where customers have been touching displays with their sweaty hands.
Doors are made in door factories, at low cost, and are distributed through door merchants, joinery businesses, and kitchen fitting companies, who adjust the retail price to suit the pockets of their customers.