Epitaxial chips are not the same as chips.
Epitaxial sheet refers to a semiconductor film grown on a single crystal material with a matching crystal structure, taking GaN as an example, a layer of GaN film with a complex structure (including n-GaN, quantum well,n-GaN, etc.) is grown on sapphire (Al2O3), which is called epitaxial.
The chip refers to the epitaxy for processing, and its main purpose is to add electrodes to the epitaxy for packaging and application. The main material of the chip is monocrystalline silicon, which is still GaN material. It can be said that epitaxy is the raw material of the chip, and the chip is to add electrodes on the basis of epitaxy (some chips also do passivation films, mirrors, and so on).