Depends on how it's implemented. Anyone using a "media proxy" will see their discord bridged media probably fail to load (outside of possible caches) after a day. Anyone who has their bridge configured to reupload discord media to their homeserver should see no change.
Yes exactly. The bridge logs into the discord server as a user.
Then it mirrors all chats from your user in matrix to the discord.
Oh matrix, every user on the connected server gets a user whose name is their snowflake. Those virtual users post into the matrix server whatever their respective discord users posts.