We are using Flutter at our company and it's far from a black box if you get stuck.

For example, try going to a TextField widget and use go-to-definition view the source source code. You can go very far down the go-to-definition hole. Even to the point where you can see where the blinking cursor is being rendered.

For several "blockers" we did have, we were able to copy/paste the Flutter implementations, rename them, and modify them to fit our own needs.