@shielaaston Chromium is essentially the rendering engine that underlies all chromium-based browsers (Opera, Chromium, Vivaldi, the new Edge, and a Google-customized chromium variant used in Chrome). All these browsers apply their own-designed user interfaces on top of the chromium engine. Depending on the flavor of browser and its design philosophy, the user interface of a given browser may or may not differ a lot visually and functionally from Chrome. Some Chrome features may not appear in Opera or other chromium-based browsers because of Google's Chrome customization. At the same time, many of the functional elements of Chrome do exist in the other chromium-based browsers because of the largely-shared rendering engine.
Opera and its sync functionality are as secure as any other chromium-based browser, particularly since Opera makes an effort to keep as current as possible with chromium engine security updates. I'm not personally a sync user nor a Google docs user, so somebody else will have to address those questions.