Most command block creations use armor stands which are wearing items with custom models, but maybe what you saw is really an entity with a custom model.
You see, you can apply a custom model to an item by overriding the items .json file with a resource pack. What's even better is that this custom model can be set to appear only if certain criteria are met, for example, you can set a custom model for a pickaxe to appear if the pickaxe is damaged 50%
Here is a video on how to do that:
But again I'm not sure if what you saw is based on this, What I do know is that all of these people use resource packs to load these models and you have to have some software to make a model in, you can't just make one with commands