I took the occasion to check what is the mechanism of overriding.

Below what myself found in Bitbake User Manual, ch. 3.3. Conditional Syntax
(Overrides) "You can use OVERRIDES to ... conditionally append or prepend
the value of a variable."
This is the point where I wonder why overriding is discussed in context of
append and prepend.
Which element of appending and prepending operations has the nature of
overriding other things?

Not sure if I understand the question, but maybe you're looking for this example (given in section 3.3.1):

KERNEL_FEATURES_append = " ${KERNEL_EXTRA_FEATURES}"
KERNEL_FEATURES_append_qemux86=" cfg/sound.scc cfg/paravirt_kvm.scc"
KERNEL_FEATURES_append_qemux86-64=" cfg/sound.scc cfg/paravirt_kvm.scc"

Thanks,
Chris