I'm reading Next.js fetching data part of the documentation and one question came to my mind.

Next.js can fetch data with getStaticProps, getStaticPaths, getInitialProps and getServerSideProps, right?

But some happens on Build-time as I quote:

getStaticProps (Static Generation): Fetch data at build-time

When is this Build-time happening?

Is it when we run npm run build? (To build the production build)

Or when the user accesses our Next.js app? (On each request)