Will Apple send me a notice to allow me to extend my AppleCare coverage for my Mac and devices?

You may or may not get a notice to extend your applecare coverage however you will not be able to renew the coverage until the current coverage expires then you have a 30 days to purchase new coverage. All of the details and the steps are in the article

Extend your AppleCare+ coverage - Apple Support

My two 2020 27" iMacs just had their AppleCare expire earlier this month. I received an e-mail for each machine advising this was occurring with the offer of extending the AppleCare, I took advantage of the offer.