/boot/uEnv.txt and its effect on device tree

Hi,
I’m wondering how /boot/uEnv.txt can effect device tree at boot time?
does the boot-loader change the device tree according to the uEnv.txt?
or it just pass them through to kernel and kernel make the proper changes?How?

dtb=

Tell u-boot which *.dtb to load when booting.. (thus override the u-boot
eeprom lookup table)

cape_enable=
cmdline=

this get's added to the kernel boot cmdline, to make overlay changes on
bootup..

Regards,