How we can automate a Canvas with a starting audience sourced from a CSV.

I think what you are asking is that you want the users to automatically enter the canvas / trigger the canvas when the CSV is imported?

In that case, one simple way set a custom attribute in the CSV and set the canvas to trigger when that attribute is set to a specific value or if changed ( am am assuming you only want them to go into the canvas 1x no matter how many times they are imported).

https://www.braze.com/docs/user_guide/engagement_tools/campaigns/building_campaigns/delivery_types/triggered_delivery/#step-1-select-a-trigger-event