Google is not the payment processor. This is about the app including a link to a donation page on opencollective.com.

Other 501(c)(6) organizations you might have heard of include the Linux Foundation and the Rust Foundation.

Do LF and Rust solicit individual donations via phone apps (or at all, really)?

Indeed, those are structured as corporate-funded consortiums of exactly the type you'd be very suspicious of if, say, your kernel or compiler logs started stuffing a donation link into the stream!

Note that GNU software does historically mention donations (not in the logs AFAIK), and indeed the FSF is a 501(c)(3).