I suggest watching the following videos to get a better understanding:
Agent - None
Autoscaler - None

Also please review agent docs - None

when a task is enqueued when does the autoscaler kicks in?

You're looking for the polling interval parameter as mentioned in the documentation - None

assuming i wasn't using docker, would the agent try to install all the dependencies on my local machine?

Not sure I understand what you mean. The autoscaler spins up an ec2 instance and the agent handles everything inside. If you don't use docker then it will simply install everything inside a virtual env on the ec2 instance itself.