I think it would be run on a network interface. Isn't this or an equivalent implementation that comes packaged with every OS so that you can connect to a network?
I may be wrong here and others are more than welcome to correct me.
This is definitely not the implementation that comes packaged with every OS. Every OS has its own TCP/IP implementation that usually lives in the kernel - though most are derived from BSD's TCP/IP stack.
SmolTCP could (in theory) replace the implementation packaged with an OS, or even be used completely from userspace by taking over the raw network interface.
I may be wrong here and others are more than welcome to correct me.
EDIT: Added "or an equivalent"