What are RenderObjects and how do you create custom ones?

RenderObject is the core of Flutter's layout and painting system, responsible for exact pixel-level drawing and hit testing.

Widgets and Elements are just configurations. RenderObjects do the heavy lifting. To create a custom layout or drawing that Flexbox can't handle, you extend RenderBox, override performLayout() (to calculate sizes based on constraints), and override paint() (to issue Canvas draw calls).