In the wiki, it says to do this:
git clone git@github.com:brave/brave-core.git path-to-your-project-folder/src/brave
cd path-to-your-project-folder/src/brave
npm install
# the Chromium source is downloaded, which has a large history (gigabytes of data)
# this might take really long to finish depending on internet speed
npm run init
I cloned brave-core, but there is no /src/brave folder.