Building onion websites is generally no different than building regular websites. The main difference is the underlying connection protocol and not the website itself. You build onion websites using the same tools such as a web server (Apache, Nginx, etc), the documents (HTML, css, etc), and server-side languages (PHP, NodeJS, etc). There is no special coding languages or design software. If you know how to build a regular website, then you will find it easy to build an onion website.
Setting up an onion service is usually as simple as changing one or two lines of your tor configuration file (the torrc file) to point to your web server. If you need special features such as load balancing things get a little trickier, but a simple onion service is usually very easy to set up once you've built the website.