Legacy access key

See >>> How to add a Legacy Contact for your Apple ID - Apple Support

• If your Legacy Contact is using a device with iOS 15.2, iPadOS 15.2, or macOS Monterey 12.1 or later, you can notify them via Messages when you add them, and if they accept, they automatically store a copy of the access key in their Apple ID settings. You’ll receive a notification if they decline your Legacy Contact request, and you can choose someone else.
• If your Legacy Contact is using an Apple device with older software, you can ask them to update to iOS 15.2, iPadOS 15.2, or macOS Monterey 12.1. Then you can use the Send a Message option to allow them to receive and store the access key on their device. Or you can print out and give them a copy of the access key. 
• If you add someone who doesn’t have an Apple device or can't access Legacy Contact settings on their device, you'll need to give them a physical copy of the access key. You can print a copy of the access key and give it to them, send them a PDF or screenshot of the page, or save a copy with your estate planning documents.

Note: If you use Send a Message to share the access key, it's a good idea to confirm that your Legacy Contact received it. However you share the access key, you may also want to print and save a copy of their access key with your estate planning documents.