Settings> biometrics and security> other security settings> pin windows.
Once you do that, u can open your recent apps you can tap the apps icon and click pin this app.
This will make it to where the app will stay open no matter what (you can even turn off your phone and turn it back on and you won't have a password it will simply go back to the app)
And when you want to unpin it it needs your password 👍🏻
This is good if you're giving your phone to someone to only use one thing.