It'll be a combination of nascent technologies on the cusp of viability.

First, something like this will have to be manufactured with a future process node about 2-3 generations past what is currently planned. Intel has plans in place for "18A", so we're talking something like "5A" here, with upwards of 1 trillion transistors per chip. We're already over 200 billion, so this is not unreasonable.

Power draw will have to be reduced by switching materials to something like silicon-carbide instead of pure silicon.

Then this will have to use 3D stacking of some sort and packaging more like DIMMs instead of a single central CPU. So dozens of processor chips per box with much weaker memory coherency guarantees.

Some further simplifications might be needed. This might end up looking like a hybrid between GPUs and CPUs. An example might be the IBM POWER server CPUs, some of which have 8 hyper-threads per physical core. Unlike POWER, getting to hundreds of kilocores or one megacore with general-purpose compute might require full-featured but simple cores.