Assembly of any vehicle always begins with the chassis. Unless otherwise specified, only one of each module type is required.
Modules from different tiers can generally be mixed to construct a functioning vehicle; however, multi-part components, such as treads or wings, must have all parts of the same tier.
The Assault Ornithopter has several optional modules that can give it scanning, storage, boosting, or even combat capabilities.
Apart from the Assault Ornithopter Thruster, all other optional modules share the side utility slot on the Assault Ornithopter, thus only one can be equipped at a time.