How To Erase the 2019 iMac Content
I wish to transfer ownership of the referenced iMac to a family member. How do I erase the hard drive? It has a 3.7 GHz 6-Core Intel Core i5 processor.
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What to do before you sell, give away, trade in, or recycle your Mac - Apple Support
See the section with instructions if you can't use erase all content and settings. (that feature is specific to Apple silicone based Macs).