how do I install rpm fusion repos on debian? I only found instructions for fedora and rhel https://rpmfusion.org/Configuration
Stop. You do not want to do this.. While resources published on other sites may be full of information, that information is not always relevant to you. Don't blindly follow bad advice.
The "rpm" in "rpmfusion" refers to the filetype that Fedora's built-in package management system, dnf, uses.
You want to use Debian's builtin package management system, apt, which uses the "deb" filetype.
Here is an explanation of how to add Debian's "non-free" repository
Do not follow information for other distros unless you know how to extract the bits that are relevant to your distro.
In general, I recommend following the advice from Debian's wiki or website, then debian's forums if you can't find anything there, then debian specific forums elsewhere, then other distro's wikis, then any other site in a last-ditch effort.
Now that you understand the "why," here's the "how": go back to Debian's download website and download the appropriate installation image from the bullet point that says
A larger complete installation image:
Reason being: the smaller "netinst" images are made to work generally for most people who can plug their computer into ethernet. It's made to only use the bare minimum of disk space and get the rest of the files it needs from the internet (the "net" in "netinst").
You need the installation image that come with the "drivers" (firmware) for your WiFi card already on disk, which should automatically detect your device, find the correct firmware for it, and set up the non-free-firmware repository for you.
If that doesn't work out for you, you can try manually installing using the guide on Debian's own wiki, which I found by searching for your wifi card BCM4360