Clarification of what grub-install does, versus grub-mkconfig/update-grub

[Chris Green]

I'm slowly working my way to a working 'two-versions of xubuntu'
system and I think I'm beginning to understand what the various bits
of grub do.

So can I confirm my understanding please.

grub-install installs the 'invisible' bits of grub like the MBR and
the code executed after the MBR.  These reside on parts of the disk
that one can't normally 'see'.  They are installed, by default, on the
'first' hard disk.  There only needs to be one set of these 'bits',
something I hadn't originally understood (assuming I'm right that
there's only one!).

grub-mkconfig (or update-grub on ubuntu etc.) creates the grub
configuration file and other stuff that resides in the /boot
directory.  There's one of these for each bootable [Linux] OS.

What I still don't quite understand is what happens with (as I have)
more than one OS installed.  Presumably only one grub.cfg gets used
even though there is more than one. How does the initial boot process
(starting with MBR etc.) and created by grub-install decide which
/boot it should use (and is it easy to change which /boot it goes to?).


-- 
Chris Green