Vivaldi contains Chromium, but it isn't itself open-source, by the way.

They say of themselves that "for all practical purposes the Vivaldi source code is available for audit". I would not fully agree with that either, but I guess, at that point the open-source purists have already lost interest anyways.

https://help.vivaldi.com/desktop/privacy/is-vivaldi-open-source/

It's still the same rendering engine. There are two browsers.

Is there a mobile Vivaldi counterpart? It doesn't make sense for me that I can't share history with desktop and mobile together