"Publish" button: What is actually being sent??

I have a huge uncertainty of how the "Publish" button on Power BI Desktop application works.

If I create a report on Power BI Desktop via a local file(.csv) and then proceed to hitting "Publish":

• What exactly is being pushed to Power BI?
• If the data I am referencing is a local data file(.csv), is that data being pushed to Power BI or is it just the visualizations?
• How does this work if the reports are referencing data from SQL Server?

I would not think I would be publishing 3GBs of into to Power BI.

Thanks.