Hello Tejas,
Mendix is continuously in development and one of the latest developments is the combo box.
They now use it in their models to replace the reference set selector.
In the learning path they still use the reference set selector but in your model they use the new way the combo box.
Just watch the mendix documentation about the two ways of how to enable your association/reference.
https://docs.mendix.com/appstore/widgets/combobox/
https://docs.mendix.com/refguide/reference-set-selector/
Hope this helps,
Good luck