Both the CPU and GPU cores are Apple's own design (CPU have been since A6; GPU since… A11 or so).
They have, so far, always had a new microarchitecture. M1 has the same Firestorm/Icestorm cores as A14, M2 has the Avalanche/Blizzard cores like A15, and M3 probably skips Everest/Sawtooth to go straight to A17's cores (because Ax is on a 12-month schedule, and Mx so far has been on roughly an 18-month schedule, so it occasionally needs to skip a generation).
So far, we haven't seen them upgrade the GPU cores but not the CPU cores. I suppose they could do that, but I can't think of why they would.