All posts can be commented on, so IMO all posts are also threads. Mbin just calls text posts threads for some reason.
Under the hood, the only mandatory field is the title. In addition to that, you can use either the url field, or the text field, or both.
If there is no link, it’s a text post. If there is a link, it’s a link post (but there can still also be text), if the link is a direct link to a media file (picture or video) then it’s a media post (which can also still also have text).
It definitely does.
Matrix does not have audio rooms. You can start a group video call in a text group chat, in some clients.
It currently does not compare well with discord functionality.