It's Chromium with the Google bits ripped out, Vivaldi has their own sign in/sync functionality, built in ad blocker, and custom UI. It's based on Chromium but has quite a bit different going on, as much as Brave or Edge.
They basically just want to keep the copyright to their UI. You can see the source but they don't want anyone to rip off their UI.
And let's be real, every browser (even Firefox) has closed source server side code.
Also, the comment I initially responded to was about why isn't there a European browser not controlled by "big tech"... Vivaldi is an independent company in Europe making a browser.
