A new ticket lands with the department you chose, marked with your priority and, if you set one, the related service - which is why setting it matters: the person answering opens your ticket already looking at the right product.
Replies arrive two ways at once: on the ticket's page, and as an email to your account address. Answering the email or replying on the page are the same conversation.
A ticket stays open while the conversation is live. When it is answered and done, it is closed - and a closed ticket is not a locked door: replying to it brings it back.
Attachments can be added with any reply, not just the first message. When support asks for a screenshot or a log, drop it into your next reply on the ticket page.