Data journey - security

Hi There,

I am implementing PowerBi for some of the reports that a Financial company produce (live) in excel.

I am using Power BI Desktop on a network SQL instance does the data ever leave that environment and reside in the cloud? 

Now if I want to publish these reports on PowerBI app and mobile, to allow the management to see them. How does the data travels to PowerBI app and mobile? 

After the reports are published it will residen in the cloud? can you please explain me, how secure is the data there, should I get PowerBi pro before to be able to get security for the data ?

I do not have a clear understanding of the data flow in this situation. Thank you