For qutebrowser and Falkon, that's QtWebEngine, which you could call a fork of Chromium I guess - but it's more of a stripped-down Chromium with a stable API on top and a couple of patches. QtWebEngine is maintained by multiple people employed full-time by the Qt Company: https://wiki.qt.io/QtWebEngine
For Gnome Web, that's WebKitGTK, which is an official WebKit port (i.e. part of the upstream WebKit repository maintained by Apple and others) and also has been actively maintained for the past 13 years or so.