Yes, technically speaking ARM is RISC, just a different flavor of it from RISC-V. They’re effectively siblings. x86 on the other hand (and AMD64) are CISC processors. CISC provides compact programs at the cost of a more complicated (and therefore more power hungry) CPU. That said this is a gross oversimplification and no modern CPU is entirely RISC or CISC under the covers. Both ARM and x86 end up looking quite similar to each other when you dig into them, with x86 producing microcode from its instruction set that is effectively RISC, and ARM introducing some decidedly CISC looking instructions.

The reality is the relative power hungry-ness of the architectures doesn’t really come down to RISC vs. CISC as much as it does x86 providing backwards compatibility to literally decades of bad decisions. If x86 could jettison backwards compatibility and ditch all but the latest and greatest of its instruction set it would be able to compete watt for watt with ARM easily, but that’s a tradeoff customers are unwilling to engage with as it would render large swaths of software incompatible.