One way to think about the fact that “the mantle is a solid but it flows” is to think about ice. Ice is a solid, just look at an ice cube, and yet a kilometer-thick glacier flows.

Like ice, the mantle isn’t perfectly solid. But if you compare it’s viscosity to that of a glacier, you find that the mantle is roughly a million times more solid than ice.

https://www.nature.com/articles/s43247-022-00385-x/figures/2

(Ice is also a non-Newtonian fluid, so a precise comparison isn’t possible.)