From the website > The image boorus are longstanding web databases which host large numbers of images which can be ‘tagged’ or labeled with an arbitrary number

From the website > The image boorus are longstanding web databases which host large numbers of images which can be ‘tagged’ or labeled with an arbitrary number of textual descriptions; they were developed for and are most popular among fans of anime, who provide detailed annotations. i.e. it's pulling from a public website which crowdsources the tagging