It's funded by donation to Reproducible Builds project, and donation is managed by Software Freedom Conservancy (free software organization which also manages donation for Git, QEMU, Samba, and Wine). It's not like they are paid by their employers.

You can donate yourself. It is tax-deductible in US.

They include logos for donation larger than $25,000. Logos are here: https://reproducible-builds.org/who/sponsors/. Top sponsor is Google Open Source Security Team.