Doesn't look like a valid ARN-setting up AWS credentials in the serverless console

I am new to Serverless and when I copy and paste my ARN from AWS IAM, it tells me that.
In name, I put the User name: serverless-python-myname
Then for ARN copy and paste:

I tried dropping the arn: prefix and other prefixes before “3…” but that did not do it either

But I created my user on the serverless platform this time ignoring ARN and just using my access key ID and secret access key. Is this okay? When do I need the ARN as well?

I was able to deploy Hello World Flask App following the first part of this:

That is so strange how automated it is. I only installed serverless with the npm command so it found my aws credentials folder on my Ubuntu OS? I never did any credential set up at all from my command line locally. I only set it up as described above on the serverless console but it does not seem to have looked there since my app is not listed (it is listed in my AWS console but not Serverless console).
Can someone point me to the documentation that goes into detail of how I was just able to deploy so easily like I did when trying out Zappa for Flask? I like Serverless but do not like being in the dark about its inner workings.

In my codes there are times when I don’t exactly know what the account ID is going to be exactly, especially since we have many accounts. In my specific case I am trying to build the ARN for a policy before actually submitting it to AWS. When using Cloudformation I can simply use AWS::AccountId, however in this case I’m not using it. Was wondering if there is some magical Speed Test way of omitting the account ID and then AWS simply assumes the account ID as the one it’s in.