Dynamics 365 Portal - Data flow and security

Hello, 
MY question is regarding how the data flows between the portal and the dynamics 365 (Database). Basically, I want to know, when submitting a form on the portal, with certain fields in it, what is the back end process. This is for security purpose, I just want to know if the data is stored at all at portal level (Or on a separate portal data cloud, azure...) or the portal is just a front and when i hit submit, all that is happening is the creation of a record on dynamics 365 level? 

Any feedback or documentation would be appreciated. Feel free to get Techy and write details

Thanks
Charbel