Blink the web engine Chromium is using is a fork of the WebCore component of WebKit,which was originally a fork of the KHTML and KJS libraries from KDE. So it is other way around. You are right I the sense that the biggest contributor to WebKit is Google.

I did witness the transaction from 3.5 to 4 and from 4 to 5. Yet I am super excited about 6.

For KDE development I would recommend the KDE Neon developer edition as it has all the tools you need to get started.

It is still updated judging by the git commits. Btw Konqueror gets new releases as well.