Some internal perspective - IAM has maybe thousands of options but fundamentally it is "what does this role have access to doing (action + resource)" + "who has access to this role". That is really it from a 10k foot level.

IAM is great because it applies internally just like it does externally. The internal AWS team don't get more access than you do, and if we get access to do certain thing on your account to perform specific service that's because you have a service principle in your IAM trust relationship that allowed us access, that you can see, and audit. For instance, lambdas have lambda role because you don't want lambda service just reading your S3 buckets because "we're AWS we automatically get access", you can absolutely see and control access, even if it is internal to AWS.