Hyvä Checkout vs React Checkout
In a nutshell, the two checkouts are not related in any way.
Hyvä Checkout is a commercial product, actively maintained by Hyvä Themes.
The React Checkout is a community-maintained open-source project without official support.
The reason for the confusion between the two checkouts is that during the initial release, the React Checkout was named Hyvä Checkout. Also, some third-party integrations refer used to refer to the React Checkout with that name.
However, it has been renamed to React Checkout for a number of years now.