It doesn't matter. As long as they implement the core RV64GC instruction set correctly, standard software will run, and if they implement privileged architecture 1.10 or later correctly then the standard Linux kernel will run.
Vendors are free to use custom extensions to implement the bootloader and SBI. Normal code and even the OS won't see that. They can also use custom extensions in hardware drivers. Even in libc, if they want, though that gives more maintenance work.
Vendors are free to use custom extensions to implement the bootloader and SBI. Normal code and even the OS won't see that. They can also use custom extensions in hardware drivers. Even in libc, if they want, though that gives more maintenance work.