BTW, why do I have a browser window? Is it because Iam using Android, or I am running nodebb 1.14?
It's because you're running 1.14 actually. The actual scripts for installable PWA only appeared in 1.15.
However, browsers are confusing so they also offer the option to "add to home screen" any website, but instead of actually considering its manifest for how it's supposed to run and using the service worker properly it will be just an icon to open that website in the browser.
I would've recommended checking this website to see how it's supposed to work, but it seems to be broken in 1.16.2 unfortunately (mentioned in this issue - service worker was moved to its own directory so only paths starting with /assets/src/ are in its scope.)