Most of the code is hosted in the Firefox repository (we call it mozilla-central, often abbreviated as m-c), in the devtools folder. Development of some pieces of the tools is happening in GitHub, on the firefox-devtools organisation.
Code in m-c takes longer to obtain and build, as it involves checking out Firefox's repository and installing tools such as a compiler to build a version of Firefox in your machine.
On the other hand, the repositories in devtools-html are more straightforward if you're used to the GitHub workflow: you clone them, and then run npm install && npm run or similar. Roughly, you can work with each repository individually, and we periodically generate JavaScript bundles that are then copied into m-c.
Even if you only want to work on a tool whose code is on devtools-html, you might still need to go through the step of getting and compiling the code from mozilla-central in order to do integration work (such as updating a tool bundle).
From now on, this guide will focus on building the full DevTools within Firefox. Please refer to individual project instructions for tools hosted in devtools-html.