How do bluetooth devices show their battery percentage in the settings?

Question

I was connecting my Bluetooth headphones to my computer when I saw the battery level of my (also Bluetooth) mouse in Settings (my headphones can be finicky when I try to connect them, so I often have to access settings to get them to work). I wondered how the coding for that would've been done, but a quick search showed up with tutorials of how to see the battery percentage, not how it works.
So, if anyone knows, could you try to explain it to me? Depending on the complexity, I may not be able to understand, but I'd at least like to try.