[Thor Joachim Haga]

One has to mark the text one wants to quote, press ‘copy’, then ‘paste’ into the new post, then mark the text again and press “B-QUOTE”.

It’s fairly easy on a laptop, but probably requires some dexterity on a phone. Here’s how to mark text on an Android, according to Google:

To mark (select) text on an Android phone, long-press (touch and hold) any word, then drag the blue cursors that appear to highlight the desired range. A menu will pop up allowing you to Copy, Share, or Select All, with app-specific options like highlighting in document editors or copying in browsers.

Basically the same as on an iPhone.