If you want to login your account in youtube premium apk you need to download micro G
Simply if you just need youtube premium download from below. But for some videos like 18+ content in youtube you need to add account so to get all you subscription also download micro g
What is microG
microG is a free and open source program to accept applications developed for Google Play Services to work on systems, where Google Play Services is not available. If you’re not using Google Apps on your Smartphone, we’re sure that this tool might be an effective tool for you. This is a strong tool to retrieve your secrecy while working with the core features of Android.
microG APK was created by Marvin W who presents as MaR-V-iN in XDA developers and it is under progress. But it is a very useful software for people who are running without Gapps.