You can like any post you see in the feed, and open its comments to see what others have said or add your own. Comments on Chillverse are a simple flat list under each post rather than nested reply threads, so everyone's comment sits at the same level in the order it was posted.
Occasionally you might come across a post where commenting is turned off — this is set by whoever created the post (often used for official announcements), and you'll simply not see a comment option on posts like that, though liking still works normally.