I am curious why one of my races set as a "race group" appears under a race as a subrace/varient option while another one appears as a seperate entry??

I have a sub race of kobold and orc that appears under their origin race as a subrace/varient but I have a subrace of centaur and warforged which do not.

I'd love any support regarding this!

Because you set them to appear in those groups. It's a dropdown in the creator you picked.

Is it something you can do retroactively or you need to do it AS you're making the race from the very beginning?

It's right under the Description field.