Circuit Assemblies are simple, pre-made circuits that you can incorporate into your 3D designs. They even have a corresponding “hole” object so that you can cut out a void big enough to assemble them when printed.
When you download your design for 3D printing, the electronic components, such as the LED, vibration motor, slide switch, or coin-cell battery will NOT export. But they will leave a space, into which you can insert the real ones. That means that you can create a design with everything in place without having to delete the components before printing.
See these Instructables for where to get the electronic components for the Circuit Assemblies and how to put them together: