We only rely on it for advanced DIY functionality. You can build a Hub site without code. This is an improvement from when I first started and you had to use markdown to build your whole site layout. Many of the features that used to require code in the early days of our site builder are now represented by actual layout cards and can be filled out through form fields.
People do make some really nice sites using HTML & CSS, but that could be done with or without Hub. It is more related to their knowledge of HTML & CSS. I don't know off the top of my head if ESRI offers that type of training, but there are a lot of free resources on the web for learning the basics of HTML & CSS. I like https://css-tricks.com/guides/
If you look for the csstricks tag on some of my posts, I've published several posts with code snippets for some types of stylized cards and if you want basic components like buttons, we're starting to transition to the Calcite framework. (My one caveat about using these is that they are something called web components, which won't mean much to you at the moment. But in a nutshell, web components are easy to get up-and-running because they have a bunch of built in configurations, but they are very hard to style visually because they don't support CSS overwrites.)
While we do use Bootstrap for our grid, it isn't really necessary to know, because the drag and drop editor can be used with grids and cards to automatically handle reflow. If you want a more thorough explanation of how the grid works, I give an overview of it in this post: https://community.esri.com/t5/arcgis-hub-questions/how-to-get-all-cards-in-a-text-box-to-dynamically... with a link to the Bootstrap documentation.
You will not be able to do javascript within our application, because while our app is built using lot of javascript, embedding third party javascript is a security risk, so we don't allow it.
