They didn't say, but since their A17 is 3nm this is almost guaranteed to be 3nm. The 3nm process is the big improvement, the M1 and M2 were TSMC 5nm and 4nm which was basically the same thing.
The performance improvements here is roughly what you would expect if you knew the specs of TSMC 3nm chips.
They DID say it, right at the beginning. All the M3 chips are 3nm.
The A17 Pro benchmarks are extremely disappointing, so I don't think 3nm is actually doing much.
Significantly faster absolute single core CPU performance than a 12900K is pretty good and not disappointing at all. I find the improvements in A17 Pro substantial.