How does /usr/bin/uname work in plain english?

Since the switch to git I've been wondering how /usr/bin/uname works.
The man page is thin on details and uname.c is far too subtle.

For example, on my test box uname -a reports
FreeBSD www.zefox.org 13.0-CURRENT FreeBSD 13.0-CURRENT #7 main-c255937-g818390ce0ca5: Wed Jan 13 16:42:12 PST 2021     bob_at_www.zefox.org:/usr/obj/usr/freebsd-src/arm64.aarch64/sys/GENERIC-MMCCAM  arm64
which seems to replay git nomeclature.

However, uname -KU reports
1300135 1300134
which is admirably readable, even for me. 

Is there a natural language description detailing  how 
uname -KU outputs are computed, and roughly what they mean? 
I've noticed that different sources sometimes produce the 
same values, so the level of detail is less, but might suffice
for initial reports to the mailing lists.

Thanks for reading,

bob prohaska