They use ARM cores designed by the Arm company, but the complete chips are designed by AWS/Microsoft.

Ampere had previously used cores designed by Arm, but their latest CPUs (which do not impress much) use a custom core, like the Apple and Qualcomm CPUs.

I know, but I can't buy a Cobalt or Graviton workstation. Ampere has been the only way I could lay my hands on a nice workstation-grade ARM chip (unless you count Apple, but they also don't sell chips)