Hi there @Ruby1414, I hope you're doing well.
There is no official "hidden contacts or messages" however, there are ways to hide them.
For example, if you're looking for contacts that might be hidden, you can go to the Contacts app>Press menu on the top left>Press the source on the top left. You'll then be presented with a list to choose what contacts to show. If the "All contacts" option isn't selected, then only the contacts listed within the selected source will display, making the other sources/contacts hidden.
There is also the Secure Folder option. In Secure Folder, there is a contacts app. Contacts in there could also be hidden from the main Contacts app. The hard part is, sometimes the Secure Folder isn't always listed as Secure Folder and/or it may be protected by a password other than what is used for the lock screen.
As for hidden messages, I assume you are referring to text messages. There is no hidden option in either Samsung or Google Messages apps. There are some ways though to make them appear hidden.
For Samsung Messages, you can delete a message thread. That deleted message thread can then be placed in the "Trash" in the messages app and can later be restored. This could be used to "hide" messages if desired.
For Google's Android Messages app, there is no trash option however there is an archive option that will move the message threads from the main page and move them to the archive page. Archived threads can be moved back at any time. This can also in a way be used to hide messages.
While there is technically no hide option for these, there are ways to get it done. I hope this has helped you in any way.
Take care.
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