How do I insert line breaks in messages ?

It is for simple messages, not "War and Peace". If you need to send a lengthy post with line breaks, formatting, etc., they have this new tool called "email" which may be very helpful to you.

Not necessarily. The fact that you failed to understand it does not make it so.
There are many issues that need a lot of work on the site.
I do not think that the ability to format phone app messages with html or a word processor is high on the list. It's not a matter of simply enabling html. If they did that, people would start doing all sorts of funky things.
As it is now, a text block gets the message across.