Automatically built from source by GitHub Actions. The file "source_commit" contains the corresponding Git commit (SHA-1) that was built. As such you can validate the binaries by diffing against your own build of said commit (using the same compiler).
Files in this "binaries" branch are all licensed under Apache License 2.0, despite some of them lacking a notice as per "APPENDIX: How to apply the Apache License to your work" of the Apache License 2.0.