You can test this locally yourself with mitmproxy, opensnitch, or whatever.

You can try building the (supposedly) open-source apps you use from source.

Everyone opining here should MitM themselves every now and then. If not for your own security then maybe to make sure you're not participating in psyop when opining online and resharing hearsay or old truisms.

Because they are open about including closed parts. Its not a FLOSS browser.

Probably because they update the browser way more often than that

The difference is Chromium feels like Chrome if that's what you want to use and trust, it does not feel like Vivaldi and that's basically all that's provided here.