That Interactive Legend in Instant Apps is pretty neat. Wish you could use it in Experience Builder? Now, you can.
This Widget is designed to import the Interactive Legend Component used in Instant Apps. The Instant Apps Component library can be found here.

To use: Navigate to the folder in your terminal and run npm install --save @esri/instant-apps-components-react.
This Widget supports the Compact, Feature Count and Zoom To options from the Instant App.
Honestly, I'm not very pleased with how this came out. I found some issues with how this works and the design kind of clashes with the other Experience Builder Widgets, but because it's being imported as a complete component, I can't do anything to really improve it. I may eventually try to build my own version.