3D UI in Spline is made by adding objects like shapes, text, or images, then adding colors, light, and animation to make it look interactive. After the design is finished, the project is published to get a link or embed code. The link is then added into Framer using the Embed feature, so the 3D design can appear on the website. Usually, Framer is used for the website layout and text, while Spline is used for the 3D visuals and animations.