While there are two sizes of Apple watches, the hardware is similar for each. Apple has not disclosed the complete watch specifications, which forces us to rely on the little we currently know.
The following image shows the internals of the Apple Watch. The images were taken after dismantling the Apple Watch, from https://www.ifixit.com/Teardown/Apple+Watch+Teardown/40655.
The following image shows the reverse side of the Apple Watch: