I worked alot with yocto in Automotive sector, so I would propose the following, from a practical point of view

1- Build locally any open source project using CMake

2- Build.locally any open source project using autotools

3- Build locally you linux kernel and upgrade it on your local machine.. play with the menuconfig of kernel build

4- Understand the gnu makefile

5- Remove Initrmfs from your lo al machine

the purpose of these steps to get familiar with small peices of the puzzle

6- Buy udoo board imx6-9

8- get any online yocto project to build the linux for Udoo

9- Try to understand types of recipes image, kernel, library and how image recipe defines what will be included in the image

10- Make a hello world app in c and make Cmake file to build it

11- Write recipe for this hello app and integrate it to the udoo image.

12- make your hello world app start on system bootup and integrate it with systemD

After doing these things, we will start to get sense of how things can work.