They’re designed to accept a standard POG, but I make them with 3D printed POG size caps. It’s a snap in fit, not embedded, so people can swap them out easily (just poke an allen wrench or something through the axle hole).
Here’s one with POGs:
https://forums.yoyoexpert.com/t/what-are-you-throwing-2025/416235/2076?u=minerj
Edit: I would have shown some of the ones with custom POGs I had made, but they all have either business names/logos or the names of the bride and groom for the wedding favor yoyos, and I don’t want to dox anyone. And if you need custom POGs made, Gabriel at Custom Pogs is fantastic. Pretty sure @Raindog3030 mentioned that TRT uses Gabriel for their POGs.
If you look in the clear bin in the background you can see some without the caps, and stacks of caps on the right:
