I want to know which plugin is used to nest replies in replies?

[merefield (Robert)]

@Zard_W

so does it need to install plugins?

You can try this Theme Component

Quick Quote Theme Component

Repo: GitHub - merefield/discourse-quick-quote: A Theme component to change the behaviour of the Post Reply buttons to quote a Post automatically in a single click. What it does Simply: it changes the behaviour of the Post Reply buttons (not Topic reply) to quote a Post automatically in a single click. There are several settings to refine what is quoted and when. You can even specify nesting of quotes for that ‘old skool’ feel. Because the automated quote is delivered to the Composer, you …