For CUDA specifically, there is a fairly large set of sample code that used to be installed when you installed CUDA. But now I think it’s on the Nvidia GitHub page; you’ve got to download it yourself.
The Nvidia dev blog has some easy to follow tutorials, but they don’t get very complicated.
Nvidia also has a learning platform which offers fairly decent courses at a cost. You get a certificate for finishing.
You’ll find some books out there with good reputations. Ultimately, this is an area that leans heavily toward paying money for good quality learning materials.