While there are some open-source RISC-V cores and there are some easily available computers that use open-source RISC-V cores, most RISC-V cores are not open source.

For using an ARM core, you must obtain a license from Arm and you must pay a fee for every product that incorporates the licensed core. Moreover, the Arm license may have various restrictions on what you may do with your products that incorporate the ARM core.

With RISC-V, you do not have to pay for any license for RISC-V, even if you still have to either pay the development costs for your own RISC-V core or pay for a license to use a RISC-V core designed by another company. For some companies it may be more important that there are less restrictions, so they can design a custom variant of RISC-V, with proprietary extensions.

Except for the business model, there are no advantages of RISC-V over ARM.

There are also other open-source cores besides those that implement the RISC-V ISA, for example there are several open-source POWER cores.

If you are a silicon designer or a fab, RISCV costs nothing to license. People think this might cause more production-grade open hardware being introduced to the scene, but I don't think so. Anything that is worth selling to the actual customers will be closed source and may include security processors.

As a user no difference than any other architecture. You are completely bound by the quality/performance of the tooling, the compilers and the actual hardware

I think open source is really the big advantage. ARM is in the news every other day for some licensing shenanigans, possibly a lot of manufacturers are looking keenly at this hoping to one day get away from ARM.

There probably are not too many at this point.

What would be awesome is to have an open and standardized NN accelerator to go with RISC-V, but that is a dream.