The harvester(s) are of course built in. There are some pasives - one resistor for a master bias and a handful caps. The antenna is taken directly in - or solar cell type.
How low power is it - it all depends is the correct answer. The chip is tuned for a few different application scenario:
1. DVFS mode: Here you select a clock speed/voltage your application require - and you will get the associated voltage/clock speed in return. You can doctor your code with calls to have various performance levels.
2. Deadline based: You select a deadline for the work to complete within - and the device will throttle by setting clock speed to match this requirement. The voltage is matched for the selected clock speed.
IO voltage is maintained from a seperate regulator - so internal logic voltage can vary with stable IO voltage.
Mask rom + flash...why? Maskrom for some secure boot stuff, stdlib and math - it is less energy per fetch from mask rom vs. flash. Ideally everything should be in ROM - but that makes it hard for the end user to add his/her application