The German communication portal "Teltaref.d" stressed the necessity of deleting personal data from the smartphone or tablet before selling it or giving it to others, as the SIM card and memory card are initially removed, and all user accounts such as Google account are deleted.
And since it may not be possible for the user to remove data from the phone memory in a reliable way, German experts recommend encrypting the memory so that it becomes difficult for the subsequent user of the smartphone to access the stored data.
The encryption function can be found in the Google Android system from version 4 onwards in the Settings menu under the "Security" item.
And for new versions of the Android system, the encryption function is effective by default, and you do not need to do anything.
In the next step, you must return the phone to factory settings. In Android devices, this option is found under the "Backup and reset" or "Reset options".
As for the mobile devices equipped with the Apple "iOS" system, this function is found in the settings menu under the item "General" and "Reset".