I have worked on airgapped systems, this is annoying, and generally only done in a classified environment.
Almost all desktop OSes, development environments, etc... expect an internet connection nowadays. For updates, package management, etc... It includes security software. Some systems work well with local mirrors, others, not so much. You can't even do things as simple as an web search. And no remote work, obviously.
Usually, on the side, you have a second computer that is connected to the internet and use a USB stick to transfer stuff back and forth, with varying level of security during the back and forth. You may have a third computer in the middle for instance, with a variety of anti-malware tools on it, to analyze the content of the USB stick before transferring its content to the airgapped network, it may also involve some paperwork.
And there is more than manufacturing and marketing. There is also HR, which may be even more sensitive than manufacturing. It is not my thing, but I guess that managing a payroll without internet access is not an easy task, with banks, government and tax services, and general communication happening mostly online.
And yeah, forget about cloud services like Google Docs and Office 365 (that part should be obvious), but even the regular Office suite is a hassle. In fact, you should probably avoid Windows and Apple in general. So, LibreOffice and Linux I guess, which is great, but that's not what most people are familiar with, and not the platform of choice for management software. LLMs? Lolno.
Because cloud services are out, IT should be done in house, and competent sysadmins are not cheap. And you need your own servers, offsite backups, etc... with the appropriate level of physical security.