You mean you've been following the projects here instad of the the actual course itself? If that's the case... well, as far as I know, the projects are the same up to SnowSeeker. Paul updated them for iOS17 and the links seem to point to the same place as the individual days in the course. But you probably missed the milestones, the apps that you build youself every few projects. I'd definitely recommend to go back to them and do them. It's important that you actually write some code yourself, not just follow along with tutorials. Also, you probably missed Day 48.
Other than that you probably haven't missed much. Project 20 and 21 are beyond the scope of the course, it seems. But I'd recommend you do Project 20 anyway. As for Core Data, you'll probably be a little lost there if you've been following the previous projects. Core Data is basically the predecessor of SwiftData. Or more accurately, SwiftData uses Core Data under the hood but is much more intuintive and simpler to use and understand. It's meant to replace Core Data. Which is why Core Data haven't been included in the couse at all. However, the downside is that a lot of projects still use Core Data. It's been around since forever ago, and SwiftData still doesn't have the full functionality of Core Data. Essentially, SwiftData is the future and Core Data is the past. It's up to you if you want to learn Core Data anyway.
Personally, I'd go and build some actual apps myself and tried to make use of SwiftData. If you run into something SwiftData can't do (and it can do a lot more than was included in the course, see here), then go check out Core Data.