I have a resource in aws which is not deployed by pulumi
but i want to use the existing resource arn from aws in my current pulumi deployment (i think it is called import aws reources or something not sure)
If you don't want to use Pulumi to manage the resource then you should just hard code the ARN. ARNs are intended to be as long-lived as the resource, so if it's an always-there resource, it's appropriate to just hard code it.
If you want to query other properties of the resource, all resource classes in Pulumi have a static