[extrapickles]

In general the slow part is getting the data out of the ADCs, so by interleaving multiple you can increase the speed. To reduce costs, if you have a repetitive signal, you can trigger the ADCs at different points in the waveform to build up a better idea of what is going on.

For scopes that can do these high sample rates they frequently use two lasers and have the scope capture the beat between them as a demo of how fast they are.