On page 301+ you see for every vehicle and drone a sensor rating, thats the base sensor it has when you buy it.
On page 277 in table "Sensor Packages" you see how high the rating can be for Sensors in a vehicle/drone differing by size. In the table above called "Sensors" you see the "Sensor Array".
Now you can replace the "Sensor 2" default rating of a medium drone for example with the maximum possible "Sensor 4" rating for 4000Y extra.
Now you can pick 4 Sensor Function options (found on page 277 as well). For example Ultrasound, Olfactory Scanner, + 2 more.