Well, the one good example I have from a few years ago was a relatively simple ADC PCI card. I did have documentation from the company. They had a Windows driver, but for Linux, their advice was direct memory writes (which requires root privileges). I didn't want the application to run as root, so I wrote a Linux diver for the card.

You need to have the datasheet.

These are usually a Google search away and the way people develop drivers even in professional settings.

For simple devices (e.g. I2C/SPI sensors) the functionality, number of registers, and procedures are usually very limited and it is not difficult.

For simple devices probably user space is just fine. i.e. https://www.kernel.org/doc/Documentation/spi/spidev Usually only if you need an IRQ or something does it require kernel space.