The source is written in C. The provide a docker container that lets you simply download the repo, make changes, compile and then load to your flipper.

I hadn't actually written an original line of C code since the 1980s. After a couple evenings of following one of the tutorials, I was able to create some new functionality to a button that currently doesn't do anything.

At least one person on the Discord (Flipper Devices, I think) watches when someone mentions how they think it would be cool to make changes to the firmware, When that happens, they upload a PDF of the K&R book "The C Programming Language" to the chat.

The firmware is open source, and the manufacturer provides a really smooth desktop app for managing different versions.