XUI Views is a b4x library (https://www.b4x.com/android/forum/threads/100383/#content). The same b4xlib library is compatible with B4A, B4i and B4J It is a collection of custom views and dialogs. Everything is written in B4X. The source code is included inside the b4xlib file, which is a zip...
You may also have a look at the B4X Documentation Booklets.
And more specific: B4X CustomViews, this booklet explains how to make CustomViews with examples and source code.