Getting Started with OH3: rewriting the tutorial - 1. Introduction and Getting Started with OH3: rewriting the tutorial - 1. Introduction might help fill in some of the gaps.
My understanding is BasicUI and .sitemap files should work the same as they always have. Pages are something different and in some ways much more complex and also much more capable. There should be a way to build managed sitemaps in the new UI but I don’t think that feature has been released yet. If you’ve ever played with HABot Pages will feel somewhat familiar.