Ubuntu had (I don't know if it still has) an additional contrib section in the sources.list for binary packages from "partners" without source code available, like e.g. Spotify.
Sorry, I mixed that up. It was named Canonical partner or something like that and contained only binary packages. Debian contrib are free packages with dependencies in non-free. While non-free are packages with not DFSG compliant source code (but with source code).
Yes, since Bookworm, there is also non-free-firmware which before was located in non-free. I've skipped that for simplicity, as both follow the same rules and non-free-firmware was introduced basically for convenience.
Do you know if either of the non-free repos contain binary files without having the source available?
OK, TY. I've thought, there were just downloader packages, containing scripts to download the firmware binary from the device manufacturer and install it on the system, like e.g. the one for the Broadcom wireless driver.
That would be contrib - free software that downloads or relies on non-free software. non-free and non-free-firmware just contain straight up non-free (but redistributable) binaries.