I’ve used Linux a very, very long time. You need to reboot in most distros if there’s anything involving a kernel update or other low level services. There is a paid service that Canonical offers on their server distro that let’s you do a rebootless update, but it’s not for us peasants. And you might get away with it on NixOS and maybe SilverBlue, but I’m not certain about that.

Sure, you don’t have to reboot, but you won’t be using a new kernel until you do.