Why is Windows Update stuck at a random percentage and never finishing?

Why does Windows Update on my Windows 11 machine get stuck at a certain percentage and never move forward, even after leaving it overnight? Why does restarting the PC sometimes reset the progress to zero, and why does the update history still show it as pending?

Is this a known bug with the current update service, a corrupted SoftwareDistribution folder, or something related to the Windows Update Medic Service? And is there a reliable way to force the update through without reinstalling Windows?