If you are going to the level of the above, you go with co-location in purpose built centers at a wholesale level. The "layer1" is all done to the specs you state and you don't have to worry about it.

On-prem rarely actually means physically on-prem at any scale beyond a small IT office room. It means co-locating in purpose built datacenters.

I'm sure examples exist, but the days of large corporate datacenters are pretty much long over - just inertia keeping the old ones going before they move to somewhere like Equinix or DRT. With the wholesalers you can basically design things to spec, and they build out 10ksqft 2MW critical load room for you a few months later.

A few organizations will find it worthwhile to continue to build at this scale (e.g. Visa, the government) but it's exceptionally small.