OneUI Core VS OneUI.....

Core is basically for midrange phones! So some functionalitys on OneUI won't support the core version..

One UI is the customized Android skin installed on Samsung Galaxy smartphones. It comes in two variants – regular One UI and One UI Core.

For starters, One UI Core isn’t a beta version of One UI. It's a stripped-down and stable version of the main One UI meant for the low-budget and mid-range Samsung Galaxy phones. That means it has fewer features compared to the One UI which is mostly found running on other Galaxy phones.
Ex : https://docs.samsungknox.com/admin/fundamentals/faqs/kba-325-samsung-one-ui-core-devices.htm

• What features Added to OneUi (Not Core/Stable Version)

Contrary to popular belief, Samsung retains features such as dark mode, gestures, focus mode, digital well-being, ability to hide apps, one hand mode, etc., in One UI Core. To top it all, the overall user interface and experience is nearly the same.

And,
Good Lock App
Bixby
Easy Mode
Samsung Pay/Pay Mini
Knox and Secure Folder
Link to Windows


*In the previous One UI 2 version, there were major differences between One UI and One UI Core. But the latest version, i.e., One UI 3, added most features to One UI Core. In the future, Samsung may add those missing features in One UI Core as well.