> What if that core isn’t free

Just context switch it, like how you run 2 programs with single core cpu

It's correct to point out that you could end up in a situation where your "big" cores are all heavily loaded and your "small" cores with less instructions are all idle. That's unavoidable if your whole workload needs the AVX512 instructions or whatever, but it could be catastrophic if your OS just mistakenly thinks it does. But that doesn't seem unavoidable; see my comments further down the thread.