To enable a screen lock on your device, add a PIN, a passcode, a password or a biometric security feature to lock your device, and prevent others from opening it.

How to set up, use, and change a passcode:

https://support.apple.com/en-us/HT204060

Android

https://support.google.com/android/answer/9079129?hl=en

https://support.apple.com/en-gb/guide/mac-help/mchlp2270/mac

https://support.microsoft.com/en-us/windows/change-or-reset-your-windows-password-8271d17c-9f9e-443f-835a-8318c8f68b9c