Let's say you want to download this open source project by Roman Nurik. Here is how you can do it.
https://android.googlesource.com/platform/developers/samples/android/+/master/ui/actionbar/DoneBar
Only keep the part before the + sign:
https://android.googlesource.com/platform/developers/samples/android/
Open this new URL, and note that at the top of the page there is a hint that says:
git clone https://android.googlesource.com/platform/developers/samples/android
That is the git command you need to use to download the project (parent project, to be exact). Use it in the terminal on Mac OS to get a local copy of that folder.
