The first place you should head over to is the
Settings screen. To get there, tap on your initials in the top-left corner of the screen (on Android you can also tap the three dots on the top-left and then
Settings).
There are three settings on iOS and four on Android I recommend turning on, and a few others worth taking a look at.
Screen Lock (iOS and Android): Means you have to enter your biometrics (Face ID, Touch ID, fingerprint or passcode) to access the app
Enable Screen Security (iOS) or
Screen Security (Android): On the iPhone this prevents data previews being shown in the app switcher, while on Android it prevents screenshots being taken