For iOS: Select your pictures or files by pressing and holding them. Click on the icon with the arrow pointing upwards and then select the option "Save picture" or "Save files":
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After downloading, your pictures will be in Apple's "Photos" app. Your files are saved either under "Files" or on "iCloud".
For Android: Select your pictures or files by pressing and holding them. Then select the option "Download" via the three dots. You will then find your pictures or files in the mobile phone gallery in the album "myCloud" or in My Documents under "Downloads".