You can assign instruments to solo and section players as well as to ensembles.
In the real world, instruments are played by humans, and this is also reflected in Dorico. Before you can assign instruments, you must add players or ensembles, which may in turn also be assigned to groups if needed. If you add ensembles, it is by default not necessary to add any instruments, since these are already included when you select the type of ensemble. However, you can add further instruments to ensembles.
You can always change the initial specification and add or delete instruments.