https://phys.org/news/2018-10-illuminating-dark-web.html

In brief, dark websites are just like any other website, containing whatever information its owners want to provide, and built with standard web technologies, like hosting software, HTML and JavaScript. Dark websites can be viewed by a standard web browser like Firefox or Chrome. The difference is that they can only be accessed through special network-routing software, which is designed to provide anonymity for both visitors to websites and publishers of these sites.

Websites on the dark web don't end in ".com" or ".org" or other more common web address endings; they more often include long strings of letters and numbers, ending in ".onion" or ".i2p." Those are signals that tell software like Freenet, I2P or Tor how to find dark websites while keeping users' and hosts' identities private.