How exactly Netlify CMS works?

so I recently got interested into git-based CMS and serverless apps, but I have a confusion about the whole branch saving of the new data, for example how does it work with important data like in an E-commerce shopping which a user wants to buy an item, so the user must be authenticated first, my question is where does all the passwords of users will be saved (inside git I assume), and more important thing is that is it safe to do this, I personally don't think so, more over there is important data like the sale analytics and the yearly revenue that the site should keep in it and i don't think that these type of informations anybody would like to be seen by public or if the repository is private by the git owners Itself. What do you think? Am I right about the whole data security thing or wrong? Please correct me if so?