The original YouTube app for Android is very big and has many not really useful features. It also isn't Open Source.
NewPipe is a new and very slim and lightweight alternative. Its completely Open Source. It wraps the web page of YouTube to get the needed information.
If you use it, you'll also have no ads before the videos, so that's a much better experience.
It also has a very simple but well designed user interface. See yourself:
You have to know, that a login feature or for example commenting isn't possible at the moment.
Unfortunately the app isn't available on Google Play, because Google would ban it. But it's available on F-Droid, an free open source app store for FOSS-apps. To install it you don't have to install F-Droid, you can download directly NewPipe's apk.
Feel free to download it here: