A React component is a class or function - there is no static "data structure", except for what any individual dev might define as convention for their own application. Unless we're calling class members "declarative data structure", in which case we may need to revisit some definitions across the programming world.
And it's not really representing UI: that's the main point here. Layout is decoupled; it's only really representing front-end logic and state.
It's a far cry from an immediate mode API where you are making draw calls.