Sunflower is supposed to be the latest and greatest with Android, Google isn't always the best at keeping documentation up to date though so I wouldn't treat it as the holy grail.
https://github.com/android/sunflower
DisneyMotions is a neat library if you are interested in animations.
https://github.com/skydoves/DisneyMotions
Kotlin wise, I would still start with Kotlin Koans though, it provides some good fundamentals that you can build off of.