They run a Wireguard network between the nodes so you can have a mix of on-premise and cloud within one cluster. Works really well but unfortunately is a commercial product with a pricing model that is a little inflexible.
But at least it shows it's technically possible so maybe open source options exist.
You could make a mesh with something like Netmaker to achieve similar using FOSS. Note I haven’t used Netmaker in years but I was able to achieve this in some of their earlier releases. I found it to be a bit buggy and unstable at the time due to it being such young software but it may have matured enough now that it could work in an enterprise grade setup.
The sibling comments recommendation, Nebula, does something similar with a slightly different approach.
Sidero Omni have done this: https://omni.siderolabs.com
They run a Wireguard network between the nodes so you can have a mix of on-premise and cloud within one cluster. Works really well but unfortunately is a commercial product with a pricing model that is a little inflexible.
But at least it shows it's technically possible so maybe open source options exist.