You dont need any external app to lock apps... Follow these simple steps to lock any app(s) as shown in my screenshot: 1) Open the app that you want to lock 2) Click the App tray/ Background button 3) Click on the logo of your desired app 4) Select "Lock the app" option 5) Now, your desired app will be locked in the background!
The requirement that I have described above is for locking a app where one has to enter a PIN/Pattern /Password / finger print / face recognition in order to open a locked app.
I followed the steps given by you above. I was unable to find out where is App tray / background button for quite some time. Later, found that the screen which comes up after clicking 3 parallel lines in the navigation bar (at the bottom) is what you were referring to. Samsung calls it is as "Recents" button.
After locking the app with the approach listed by you, it is still not locking the app. When I come back to the home screen and then open the same app, it is opening up without asking the Password/Pattern or other security codes.
This is not the requirement that I have described in my post above.
I see that multiple important functions have been hidden very deep inside by Samsung OneUI software and cannot be located very easily in "Settings" screen or else the function or feature is not supported by Samsung and one has to search for another app to get the required function in Google Play store wasting several multiple hours. Take for example, function for screen capture and function for "screen off" using software icon without pressing screen off physical button each and every time which damages the power button too soon.
s secure doesnt work well. it only locks the app once. after unlocking once it won't lock again. it will lock again only if u reboot the phone. also if someone uninstalls s secure on your phone, then all the locked apps are unlocked again