It is not entirely correct, although Android is from Google, it bases its influence more on the pre-installed apps and those from the Playstore, for this reason the corresponding Google APIs can be gutted (do not forget that Android itself, as well as Chromium, is FOSS) and use apps from, for example, F-Droid. Vivaldi uses Chromium, but it’s partially degoogled and leaving the rest of the APIs as user option in the configuration. FF also contains Google APIs, for example this one from Safe Browsing, even this in Vivaldi can be deactivated, in FF it can’t. It’s not actually that important if you use Chromium or Gecko, but rather how you use it. Many forks are limited to putting their own logo as it is and others that use Chromium (Blink), but not as it is delivered, but modified so as not to send information to Google, as Vivaldi devs does very well. If I disable all Google APIs in Vivaldi, it’s not even recognized as Chromium by Google and I can’t download extensions from the Chrome Store eg. It can even impersonate EDGE and use BingChat (if I’m dumb enough to want it), which no other browser can do. Saying Chromium is Google and Firefox isn’t is absolutly wrong, In FF are novadays even working Google devs, making the browser, so its irrelevant if you use Chromium or Firefox if you use it as is, without degoogling them, both phone to Google if not, Even Webkit o <Safari do it, because Google paid for this. This is the problem, not the engine you use. https://www.cnet.com/news/privacy/a-dangerous-conflict-of-interest-between-firefox-and-google/

LineageOS and Grpahene OS exist for some androids. Lineage can be made very private and Graphene is the best for privacy out of the box.