Google has Google Assistant update with the more device compatibility. Yes, the smart assistant app got the Android Marshmallow and Android Lollipop compatibility. Find the download link & features in this post.
Download Google App v7.16 Update from here to get the Google Assistant on your phone.
Google Search app v7.16
It will replace your current voice Search now app with the new smart assistant. You just need to keep your Google Search app update and you will get the assistant in your phone in next few days.