It has a number of implementations, both fully open source ones (BSD licensed), and proprietary. This one is based on the open source Rocket Chip implementation (https://github.com/ucb-bar/rocket), which is a simple in-order design, something like ARM Cortex-M or Intel Atom. (There is also one open source out-of-order design called BOOM - https://github.com/ucb-bar/riscv-boom)
It has a number of implementations, both fully open source ones (BSD licensed), and proprietary. This one is based on the open source Rocket Chip implementation (https://github.com/ucb-bar/rocket), which is a simple in-order design, something like ARM Cortex-M or Intel Atom. (There is also one open source out-of-order design called BOOM - https://github.com/ucb-bar/riscv-boom)