They're not random, they're a French non-profit (created by cofounders of nextdns, btw). Why wouldn't people trust them? In my country most people use a DNS hosted by their ISP, and there are a lot of small ISPs all around. I suspect it's similar everywhere. How is it different?

And anyway, I trust a random French small company more than I trust Google.

Well yes exactly, the only reason you'd switch to a trusted service over a default is to hopefully shield from DNS poisoning and other shenanigans. It would need to be a proven trustworthy service, and just about anyone with a few hours of spare time and literally no cash can open a French NGO, they've got the lowest barrier for entry of any EU country. Just slapping on a nextdns logo doesn't mean shit. It's completely pointless unless it's an EU official government service financed directly by our tax dollars.

Google has a reputation to uphold, so while you can be certain they'll be datamining the shit out of your requests they are also unlikely to be direct malicious actors.