The average bystander doesn't have to care, just buy a machine implementing the RVA23 profile (standard set of extensions) and be happy.
If you're building your own embedded hardware then you determine what your needs actually are e.g. do you need double precision? half precision? vector?. Then you choose a chip implementing that. Then you copy the ISA string from your chip's documentation to the `-march=` argument for GCC/Clang and be happy.
It's not hard and you don't have to think about it unless you very specifically want to.
If you're building your own embedded hardware then you determine what your needs actually are e.g. do you need double precision? half precision? vector?. Then you choose a chip implementing that. Then you copy the ISA string from your chip's documentation to the `-march=` argument for GCC/Clang and be happy.
It's not hard and you don't have to think about it unless you very specifically want to.