Hi there @Karim_Assad,
Reading between the lines, and from your previous post here I would say:
Yes, in theory it is possible. There are no limits for users using public apps, and you can put it behind a login if it is only "public" for your users. "External users" under retool pricing have an account in your retool environment and are able to use all of your resources, have attributes, be part of your permission groups, etc etc.
Bear in mind that there are some limitations for public apps and you may find yourself a bit limited if you want to do more complex logic and/or use APIs