What's the difference between the plus button and the heart one?

[Alex11950]

Plan

Premium Family

Country

Italy

Device

Huawei P8 lite

Operating System

Android 7.0

My Question or Issue

I don't understand the difference between the two buttons with the plus symbol and the heart one: do they both save the element in the library? Do they both put like on the element? 

[user-removed]

The heart saves the song in your library as liked "Loved tracks"

With the plus you should be able to add it to a custom Playlist. Just try it, the plus wont add it anywhere on first tap.

Not exactly sure because it looks different on my phone. You are on the latest version?

[Alex11950]

I'm not on the latest version, so this could be the problem... However, when I tap the plus button, appears this symbol, but nothing seems to happen and the song isn't added anywhere. 

Thank you for your answer!