Display link or button on a post based on conditional logic

Hello. I am trying to recreate the registration functionality of this site https://www.nomonkeybusinessdogtraining.com/classes/. I have created the same custom post type “Classes” where the admin can add a class, different days and times for each class and whether it is “Open” or “Full” here https://nmbdt.713creative.com/classes/
You can see the formatting of the classes is similar with the exception of:
If “Open” is selected on the “Class” (post), a hyperlink is displayed to send them to a Registration Page under Register.
If “Full” is selected on the “Class” (post), it just says Unavailable under Register.
I can not deconstruct how the old developer did this and whether it was even through CPT UI. Can anyone point me n the right direction?

The page I need help with: [log in to see the link]