Does the official Windows installer include CUDA?
Typically no. The official installer targets CPU builds. For GPU, use a CUDA-enabled build of OpenCV (prebuilt from a trusted source or build from source).
Which CUDA Toolkit version should I install?
Pick a version supported by your NVIDIA driver and by the CUDA-enabled OpenCV build you plan to use. Keep versions aligned.
Do I need cuDNN?
Only if you plan to use deep neural network modules that benefit from cuDNN. Otherwise, it is optional.
How do I verify that CUDA works?
Check that your OpenCV build reports CUDA enabled and that at least one GPU device is detected. Then benchmark a GPU operation against CPU.