I don't know what you mean by "build". To many it means to strip down and modify the bike so it is far from stock configuration.
Unless you tell us what your bike still has, we won't know if you still have the stock components.
The stock bike has a solenoid, which is where the starter motor wire attaches. The battery connects to the other post on the solenoid.
It is the solenoid that should connect the starter motor to the battery when commanded.
If you are connecting the starter motor directly to the battery, then of course it is going to operate when connected.
FYI, there are about 12 wire diagrams on the this site.