Does anyone know how to turn it off?
I use Firefox on mobile and have never had such an issue. Might be worth a look.
Some Android versions also have a specific text editing keyboard which you can pull up instead of your normal keyboard, with arrows keys, "select", "select all", cut/copy/paste, etc. If you long press on whatever button you usually do to bring up the icons for keyboard settings, emojis, etc., the icon should be in there. Alternatively in your keyboard settings, you may have an option to enable a keyboard toolbar or similar, which will have the icon on it. Even without the zoom issue, I find it way easier to edit on phone using arrow keys than trying to accurately select the start and end points of a selection with my finger.
To answer OP, I agree with majority of other answers in this thread. Quote first, reply second. If I am replying to someone's entire post, I'll quote them and just type -snip-. If I am replying to individual parts of a post, I'll just quote the relevant sections, trimmed down as much as possible (as I just did above) and with nested quotes removed.