how do you find the subsystem of a file?

[daniel watson]

i wrote a patch that got rejected because it did not apply cleanly to
the tree of greg kh
https://lkml.org/lkml/2021/8/18/304

the file i modified is
drivers/staging/rtl8723bs/include/rtl8192c_recv.h

get_maintainer.pl gave me the list of emails to send the patch to,
and i used it for that purpose.  the file
Documentation/process/submitting-patches.rst says to see the T: entry
for the subsystem in MAINTAINERS to find the right tree to base the
patch on.  i tried searching through MAINTAINERS and found that there
are a few subsystems that start with RTL8*.

greg kh is listed a few times in MAINTAINERS, so i'm not able to find
the exact tree to start with by looking for the maintainer.

is there a systematic way of finding the subsystem, given a file?

in addition, how do i know what branch to use?  the T: entries have a
repo, but not a branch name.

thanks!
:wq, danny