If I erase an old Mac, will all the same files, documents, etc. be erased from my other devices?
If I sell my old iMac, and erase it will all the same files, documents, etc. be erased from my other devices [iPad and iPhone?]
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Here's what you do: https://support.apple.com/en-us/HT201065
Once you sign out of your Apple ID and all the other services you're signed into, your Mac is isolated.