A typical embedded board is based on a reference design from the manufacturer with changes to make it suitable for a particular application. The BSP that comes with the reference board should support all of the peripherals on that board. But, then you customize the design, perhaps by adding a temperature sensor attached via I2C, some lights and buttons connected via GPIO pins, a display panel via a MIPI interface, or many other things. Your job is to create a custom kernel to control all of these, but where do you start to look for device drivers to support all of these peripherals?
The most obvious place to look is the driver support page on the manufacturer's website, or you could ask them directly. In my experience, this seldom gets the result you want; hardware manufacturers are not particularly Linux-savvy, and they often give you misleading...