Why did they go with the 64 bit Arm core instead of an RV64 core? (Or an alternative question: why go with the 32 bit RISC-V core instead of an Arm M0?)

Does having mixed architectures cause any issues, for example in developer tools or build systems? (I guess not, since already having 32 vs 64 bit cores means you have effectively a "mixed architecture" even if they were both Arm or RISC-V)

What's the RISC-V core derived from (eg. Rocket Chip? Pico?) or is it their own design?