As has been suggested above, probably the best answer to your question is to set up a local repository that the offline system can pull from. You can then update this local repository as often or rarely as you like, as long as you update the whole thing as a set. Trying to transfer deb/rpm files to the offline system on a per-package basis would be a nightmare to support.

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By the way, in defense of this, perhaps, seemingly unfriendly system the ideas for installing any application with just one package all mean that that particular application must deal with its own security updates and can take up twice as much space and memory as well as being potentially difficult to configure.
Just google "DLL Hell" as an idea of how Windows was a real pain to install software on. I feel I ought also to point out that even Windows programs installed today may expect certain things to be installed and are not portable as they stand.

For applications that are portable there are websites for installing to USB sticks on various OSs for use on any machine using that OS

To an extent Mozilla's binary Firefox package can be expected to work on most failry up-to-date Linux distros with Gnome* installed.

*not a prerequisite, as far as I can tell, but will "just work" if it's installed.

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in addition to all that's been said already, most package managers have a "download only" option.
assuming apt - that would give you a .deb package that can be moved to another (offline) system.

but still! you have to check on https://packages.debian.org/ and get ALL depndencies and dependencies of dependencies... i have done that on occasion. usually because i needed to install something to get an internet connection..

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Nothing to stop you downloading the entire repository together with any updates to a local machine.

You could use this local repository as a source of any updates for local machines.

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Thanks for your replies. I got a lot of valuable answers which I can explore according to my needs.