eks auto mode - custom amis

You are correct that EKS Auto Mode does not allow for custom AMIs. In EKS Auto Mode, AWS determines and manages the image (AMI) used for your compute nodes. There is indeed no option in the NodeClass specification to allow for custom AMIs.

Regarding how Auto Mode selects AMIs, it's important to note that AWS specifically manages this process. Auto Mode uses Amazon-provided Bottlerocket AMIs, which are optimized for running containers. These AMIs are pre-configured with necessary components and security settings for EKS.

Auto Mode doesn't select from all public images or just any image with an "eks-auto-*" naming convention. Instead, AWS monitors and rolls out new EKS Auto Mode AMI versions as needed. These AMIs are specifically designed and maintained by AWS for use with EKS Auto Mode.

If you experience any workload issues related to an AMI version used by Auto Mode, the recommended course of action is to create an AWS support case. AWS will then investigate and address any potential issues with the AMI.

It's worth noting that this approach differs from standard EKS setups where you have more control over AMI selection. The trade-off with Auto Mode is reduced flexibility in exchange for simplified management and automatic updates handled by AWS.
Sources
EKS Auto Mode - Amazon EKS
Learn about Amazon EKS Auto Mode Managed instances - Amazon EKS