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I hadn’t seen Linux do that. How do they fully do it without ever locking or rebooting the system?
When you run apt upgrade or pacman -Syu that's exactly what you're doing. The files are replaced on your drive while everything else continues running. Generally it won't affect execution of existing software, because they're all already loaded into memory, but some software might crash or get weird behavior as they try to access their files on the drive and those files have been updated, and newly launched programs…
Windows does do hotpatching, but there's a lot of things that aren't hotpatchable. Do you really think that Windows is like "naw, we could do zero reboot updates, but prefer not to because we are so dominant in the OS space"? This would be an incredible feature for the enterprise. In fact the enterprise version added a bunch of new hotpatch support just last year, but still requires quarterly updates and only does security updates. You really think that they did all that, but decided to not do the rest because they're comfy?
Again, I haven't seen Linux or Mac solve the problem fully either, nor iPhone or Android. AFAIK even every cloud provider has to do a reboot. Would Google or Amazon or Oracle have figured this out if it was so easy? How is it that there is no actual software engineer in industry that knows how to do this, but everyone on message forums seems to? Why don't these companies just hire people from message forums?