It's mostly been a midfield 2 recently. On paper 4-2-3-1 with Mount as the 10, but he's also comfortable dropping deep into more of an 8 role so it is a bit hybrid-y.From what i saw it was mostly 4-3-3.
Anyways will be good option to mix up tactics even in the game.
Ziyech is also very comfortable doing this and it's actually a major strength of his - his passing from deep is ludicrously good. I genuinely think he and Sancho would be great together - Ziyech dropping deeper to receive the ball, Sancho / Pulisic / Tammy running in behind.