Certificate v. Rating
People tend to use the term "ratings" in the colloquial sense. That may be the cause of the confusion. E.g., you earn "your ratings." You are really earning your "certificates." The ratings state which type(s) of aircraft you are qualified to operate and/or instruct. Your pilot "certificate" and the ratings on your certificate never expire. Your currency can expire (mine has - long ago).
I think the best way to describe a "certificate" is the kind of "license" you hold. I indeed realize it is a "certificate," though I further realize that outside of the U.S. it is referred to as a "license." For example, ATP is a "certificate." Flight Instructor is a "certificate." You will have "ratings" on the certificate.
As I understand a CFI certificate, with its associated ratings, let's say you let it expire. The whole thing, including its ratings, has expired. You will have to take a practical with the FAA or a DE to reinstate your CFI. At that time, all the ratings on your certificate will be reinstated. Now, let's say your CFI is about to expire and you add an instructor rating, e.g., MEI, to your certificate. Your CFI, and all of its ratings are renewed for another 24 calendar months.
Hope that helps.