Large parts of Chrome are actually GPL AFAIK, which is one reason both Apple and Google made it open source in the first place.

> chrome is what everybody uses because google has lock-in power.

Incorrect. At least on Windows, Chrome is not the default browser, it is the browser that most users explicitly choose to install, despite Microsoft's many suggestions to the contrary.

This is what most pro-antitrust arguments miss. Even when consumers have to go out of their way to pick Google, they still do. To me, this indicates that Google is what people actually want, but that's an inconvenient fact which doesn't fit the prevailing political narrative.

> so that websites only get rendered well if they use certain APIs, so now competitors to Chrome are forced to implement those APIs, but those aren't public.

What is a Chrome API that web developers could possibly implement but that "isn't public?" What would that even mean in this context?

> google says "oh I'm going to disallow you running the extensions you like, so we can show you more ads".

And that could have happened just as well if Chrome was 100% open source and GPL.

Even if you accept the claim that Manifest V3's primary purpose was not increasing user security at face value (and that's a tenuous claim at best), it was perfectly possible for all third-party browsers (notably including Edge, which has 0 dependency on Google's money) to fork Chromium in a way that kept old extensions working. However, open source does not mean that features will magically appear in your software. If Google is the primary maintainer and Google wishes to remove some feature, maintaining that feature in your fork requires upkeep, upkeep that most Chromium forkers were apparently unwilling to provide. This has nothing to do with whether Chrome is open source or not.