When updates are available there will be a link that takes you to the Software Packages module and you can review what will be installed.
If you see a reboot message, it is no longer pending. The kernel is already installed. It’s just not running yet, because you have to reboot to start a new kernel.
Kernels are just packages like any other. The only thing special about them is it takes a reboot to start running them. Everything else restarts itself, if needed to put it into service, during the upgrade.
And, also a reminder that we use the system software package manager (for our packages, too), so these aren’t Virtualmin specific questions. The GUI is just showing you what apt and dpkg could also tell you (that’s where we get the info on Debian/Ubuntu).