I am not sure if I am comparing Apples to Oranges, but both Camunda and Step Functions of AWS seem to address the same thing: Workflows. Help me with the comparison of both, which to use when. Are they replaceable?

You are not comparing apples to oranges. Both tools are workflow engines.

As a background read, the comparison is also discussed for example here: https://forum.camunda.org/t/bpmn-vs-aws-step-function/5460.

Differences in essence:

• Process modeling language (Proprietary Amazon State Language vs. standardized BPMN supporting more language constructs from http://www.workflowpatterns.com/)
• Visualization of process models for different stakeholders (simple auto-generated for Step Functions, BPMN for Camunda)
• Architecture possibilities (Step functions are cloud-only and even AWS-only, but then of course integrated in the AWS world; Camunda is independent and can run in any environment, but also needs additional work to integrate with AWS).

As a rule of thumb:

• Use Step Functions if you have quite technical workflows that need to work only in the AWS world
• Use Camunda in all other cases, including more hybrid environments and “bigger” processes

I also described this more in https://processautomationbook.com/