This is a good tutorial of how to compile JetPack 4.6.1.
Notice that the guide is bit old so you need to replace the version number with the recent one (4.6.1/R32.7.1).
Where can I download the source?
Use SDKManager probably more easier to manage the download. There is a source_sync.sh that can automate the downloading.
I ran source_sync.sh and the sources are downloaded.
But when I build the source, it shows error: make: *** No rule to make target 'tegra_defconfig'. Stop.
make: *** No rule to make target 'tegra_defconfig'. Stop.