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.
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.
Confusingly, that page only provides the changes to the Google Chromium source that allows their UI to run. (I'm not sure it would be easy to discern this without already knowing the source is not fully open.)
Search engine deals are HUGE for browsers. They're e.g. what has funded Mozilla with many billions over the last 20 years. Mozilla has tried to diversify but everything else has pales in comparison (and the donations are basically a joke).
It scales up with usage as well. Not that Safari needed funding, but Google pays Apple upwards of $20,000,000,000 per year for the privilege of being the default for that user base.
A lot of people might think $20B is a lot to pay. But search (and “other”) account for over half (>$200B) the of Alphabet’s total revenue from all sources. It’s still a bargain when you consider how few people bother to (or are even aware of the possibility of) changing their default browser.
* on my phone, can’t inspect the tars