install, update, pkgs consistency

[Gary Aitken]

I'm confused about the synchronization between a sys install, sys update,
and packages (as opposed to ports).

It was my understanding that packages are built to be consistent with the OS,
so that once the OS version is installed, any "pkg install" is more-or-less
guaranteed to be consistent and compatible with the OS and other packages
being installed.  true?

If I install a package shortly after installing the OS, then some months
later delete the package and install it again, will the same version be
installed (assuming I have not updated the system)?

When the OS is upgraded, e.g. for security patches using freebsd-update,
does the package collection remain unchanged, or is it updated as well?
By updated I don't mean the packages installed on the machine at the time
of the update, but the packages which will be installed should a
"pkg install" be done.

If I install packages, then some months later update the system
(freebsd-update), could some of the previously installed packages
no longer function properly?

If I install packages, then some months later update the system, then
install more packages, could some of the new packages not function (or not
install) because of incompatibilities with previously installed packages?

Why does /var/cache/pkg show multiple versions, some dated after the date
of the installed system, if pkgs are supposed to be tied to the original
sys install or update?
In this case, the installed system was built on 2020-01-28,
The files in /var/db/freebsd-update are dated 2020-03-14.
But there are multiple package versions in the cache after those dates:

uname -a:
   ...FreeBSD 11.3-RELEASE-p6 #0: Tue Jan 28 09:55:39 UTC 2020  root at amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC  amd64

/var/db/freebsd-update:
      14 Mar 14 17:16 f465c37...-rollback -> install.IUk7v0
   12288 Mar 14 17:16 files
     512 Mar 14 17:16 install.IUk7v0
     800 Jan 28  2020 pub.ssl
      75 Mar 14 17:15 serverlist
      75 Mar 14 17:15 serverlist_full
      25 Mar 14 17:15 serverlist_tried
     150 Mar 14 17:15 tINDEX.present
     112 Mar 14 17:15 tag

/var/cache/pkg:
   1593004 Jan 29  2020 bash-5.0.11-518f921f70.txz
        26 Mar 14 17:07 bash-5.0.11.txz -> bash-5.0.11-518f921f70.txz
   1591908 May 13 19:29 bash-5.0.16-e0c962f411.txz
        26 Jun  3 17:24 bash-5.0.16.txz -> bash-5.0.16-e0c962f411.txz
   1592200 Aug  7 19:26 bash-5.0.17-254bcebb38.txz
        26 Aug 26 10:57 bash-5.0.17.txz -> bash-5.0.17-254bcebb38.txz

I recall seeing something about a "quarterly snapshot" of ports.
Is there a consistent quarterly snapshot of pkgs and ports, so that pkgs
and ports from that snapshot should be able to be intermixed?  If so, how
does one use it?

Finally, is there an easy way to purge obsolete files from /var/cache/pkg?
I don't see anything in man pkg indicating how to do this.

Thanks,

Gary