I'm supposed to develop Magento as PWA but unfortunately, I couldn't use 2.3 versions in my server. I'm impressed by this pwa studio theme https://magento-venia.now.sh\ I couldn't get any guidance.
Magento 2.2 and 2.1 does not have feature for support the pwa.
One of most import component of PWA that is GraphQl that does not exit Magento 2.1 and magento 2.2.
If you want to use pwa then use Magento version 2.3.