ArcGIS Javascript API is different than Experience Builder (EB). For EB, you have prebuilt widgets (web pages and widgets that reside on them) that you can select from to create custom interfaces to work with Esri data. It's easier to build things than the JS API but you are still limited in how the widgets work. You can create customized widgets and add them to EB and I saw a Github site awhile back that showed how to extend some Esri widgets for EB.
For the JS API, you need to build an application which can be just Javascript and HTML, or maybe with C#, Javascript and HTML, or similar. You could do this using Notepad, but that's very difficult. Pretty much everyone uses an Integrated Development Environment (IDE) like Visual Studio to write, compile, and publish the code. In order to have the final site 'published' you need to push it out to an internet service engine, such as Microsoft's IIS or Apache. This needs to be installed on a server that has access to the internet and you can do this in house or buy space on one already in the cloud.
EB has the ability to publish the final product to ArcGIS Online so it doesn't need IIS. An ArcGIS Javascript API application can be built to access data in ArcGIS Online, but it can't reside in ArcGIS Online. The closest thing to building something (custom) and then publishing it to ArcGIS Online is Experience Builder.
Bottom line, you can't use the code you linked to inside of EB, but you might be able to customize some of the widgets in EB based on the code you find. Need to know Typescript for that.
