Every time Google releases a new update for Android, it simultaneously also releases factory images for the new version on its website. The OTA update usually rolls out over the next few weeks, which is painstakingly slow and frustrating. Flashing the factory image is not always possible since it also wipes the phone clean of all the data.
While the OTA zip file can be manually sideloaded on a Nexus device, even finding the download link of the OTA zip file can take up to a few days. So, to avoid Nexus loyalists some headache, Google has now started posting download links of OTA zip files for Nexus devices on its website. To make it clear, the zip file contains the full ROM itself, so they weigh in at 700MB+ compared to the usual OTA that Google rolls out, which are usually around 10-100MB.
For now, Google has posted the OTA zip files for all Nexus and Pixel devices for the last couple of security patches. In case you have not yet received the May security update on your Nexus device, you can sideload the OTA manually by downloading it from the website and following the sideloading instructions mentioned there.
Download: Nexus OTA zip files