I was trying to understand this exact question when I first entered the openEHR space.
And here is my question thread: Relationship between FHIR and openEHR - #5 by Sidharth_Ramesh
And a follow up answer after some more research: Relationship between FHIR and openEHR - #19 by Sidharth_Ramesh
And here’s a video delving into this topic further: https://youtu.be/OFC6V9537tg?si=4AqNSUgUhGQYbpvM&t=81 (my latest take)
You can also check out https://www.youtube.com/watch?v=kOU2HGqK23o&pp=ygUPZmhpciB2cyBvcGVuZWhy