The difference is getting blurry. Apus have generally better communication/latency/shared resources with the CPU. The ultimate ideal of an APU is to have a unified memory with the CPU, which is the case in e.g the PS3/PS4 Despite progress in heterogenous computing (the neglected HSA), in SOCs, 3D ingerposers, high bandwidth buses interconnects and 3D memory such as HBM, the PC platform has yet to see a proper APU. In fact the M1 is probably the closest thing to an ideal APU on the market. But yes the more time pass, the more the term IGPU denote APU. AMD bought ATI because of the fusion vision, the idea that sharing silicon, resources and memory between the CPU and the GPU would be the future of computing.

An unrelated but very underrated is the egpu. Egpus are external to the pc unlike a dgpu. So you can buy a thin laptop, connect it via Thunderbolt to a rtx 3080 and enjoy faster gpu performance than allowed on any laptop on the market, and enjoy a thin lightweight, silent laptop the rest of the time. Disclaimer Thunderbolt is still a moderate limiting factor in reaching peak performance.