Back Button?? Note 8

[Android Central Question]

When I am on Facebook, YouTube and other apps. When I enter into a post and I want to exit. I have to pull down from the top to get my recent, home, and back buttons to display. When I press the back button, nothing happens. However when I select the recent apps buttons, I can close the app from there. But I have go back into the app and start all over again. I know there has to be a solution. I have Google and searched for other info but I'm not finding anything