What makes you think they "go into system apps"?
Some of Samsung's apps are available from the Google Play Store, and some are available from Samsung's own Galaxy Apps store.
Still not sure what you are asking. Once you've downloaded and installed an app, just use it. You don't need to do anything else. System apps are something else, and most apps do not somehow "become" system apps. Either they already were or they are not.
If you want to see what your system apps are, go to "Settings" > "Apps" and click the upper right three dots, and click "Show System Apps" , and the list expands. The added apps would be considered "system apps" .