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)
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.