How to comment answers under my question?

You can’t. If you want to reply to a user just add @ before their name.

Here, I’ll give an example. I’m just typing in the textbox below “Answer this question” like Mimi did above me.

@Mimishu1995—I’m replying to her.

@Cubone—I’m replying to.

And now I’m just continuing on with my answer.

Once you type the @, you’ll get a menu that drops down from the bottom of the answer box. This menu will contain the names of everyone who has posted in the thread, including the person who posted the question.

The drop down menu of names only appears on the desktop site. It does not appear on mobile devices. You have to type @ and then the name, if you’re on a mobile device.

I have a desktop option at the bottom of the page on my phone. That allows me to view/use options not available in mobile mode.