I can recommend the bootlin course to get started with kernel development. https://bootlin.com/training/kernel/

Additionally, I often use new drivers "published" on Phoronix as example. They are often quite simple. The complexity usually lies in the reverse engineering the hardware, not really in the driver development.

E.g. for the apple magic mouse: https://www.phoronix.com/scan.php?page=news_item&px=Apple-Ma...

And for corsair PSUs: https://www.phoronix.com/scan.php?page=news_item&px=Linux-Co...