Just bought the kit and build progress

The cap is at the top, not the bottom.
What you're referring to is the float bowl. The plastic donut-shaped thing is the float.
It will rattle if you shake it when there's no fuel in the carb.
The float rises with the fuel level in the float bowl and presses on a pair of fingers that in turn close the float needle valve, regulating the fuel flow to maintain a constant fuel level in the float bowl.
As long as nothing else is loose there's nothing to worry about, at this stage.

Thanks for the explanation. I thought maybe something popped off. I guess don't really know anything about carbs. lol

Glad I could help.
It won't take long to learn about these carbs, there's not much to them and there's a wealth of information on this site.

This thread, NT Carb Tuning Basics, has a bit of info and some labelled pics.

Also, srdavo's Carburetor Assembly Pics is worth a look.