Are AWS Boto3 calls asynchronous?
Boto3 is a client for AWS API, so by definition it doesn't handle the synchronous or asynchronous behavior of the API call, that's the API endpoint which define that.

You can safely assume that any API endpoint marked as asynchronous will be asynchronous in any API client. Some SDK implement methods to wait for those asynchronous calls, some don't and you need to write your own handling (usually polling regularly the state) if you need to wait the end of a task started by an API call (creating a CloudFormation stack for exemple).