
The veneers are cut into pieces and put together to form a picture or pattern. Different coloured woods act like different coloured paints, and some lighter coloured woods are dyed different colours. Have a look at the panels by Francesco Abbiati to see if you agree.
There are different techniques for making marquetry that have changed as technology and materials have developed. Cabinet-makers such as Riesener who made the drop-front desk, would have used a similar technique to this process:

It takes a marqueteur many years to perfect the skill needed to make such detailed and beautiful decoration.