If you click on Quote & Reply at the bottom it will automatically add that entire message as a quote into the reply text box
If you click on +quote at the bottom of a message, this will add a quote of that entire message to a pool of quotes that will now become available for you when you use the reply text box.
NB: unless you click either of those, the reply text box at the bottom of a thread will add a reply to the topic in general, without reference to a specific post. This is fine if you are adding your thoughts to the thread in general. If your reply is to a specific post it is a good idea to quote at least a small part of the post, otherwise people may not know which post you are talking about. Having said that it may be obvious if your post is immediately under the one you are referencing.
To quote just part of a message, highlight the word/s you want to quote and click on +quote or Quote & Reply depending on whether you want to add to the quote pool or to automatically reply to just that quote (as above)
Reactions are very useful if you don't need to write a reply, but you want to let the poster know briefly what you think. This can save a thread becoming cluttered with one word answers.
You can rate posts e.g. like, funny, agree etc via the React link at the bottom right of each post
On desktop computers you can tap to give a like, or hover over the icon to see the full menu of different ratings. On touchscreen devices likewise you tap to give a like, or hold your finger on the icon for longer to reveal the menu.