You can like a publication while browsing the Feed/Explore sections, on the cover page of the publication, and also while reading the publication.
Long press on the publication no matter where you are to pull up the context menu.
Drag your finger (or the circle that appears) to the heart icon and it will be highlighted in Orange.
Release your finger and you’ll see a notification that says ‘Publication Liked’.
Also, from the cover page of a publication, tap on the heart icon in the menu at the bottom of the publication. The icon will get filled in, and you’ll see a the pop-up notification “Publication Liked”.