Hello Hagen Fu,
Yocto builds a customized Linux distribution for the i.MX8QXP Board (other NXP boards are also supported by the BSP release, and the Community BSP supports even some third-party boards). This includes the Linux Kernel and Uboot.
However, Yocto fetched the source code on-demand, which is why you won’t find the source code on the sources folders of the Yocto BSP.
For more details on AHAB please check Application Note 12312 (link below):
https://www.nxp.com/docs/en/application-note/AN12312.pdf
I hope this helps!
Regards,