Episode doesn’t thoroughly check and sometimes they have no way of knowing, so even if they approve, you still don’t actually have permission to use it unless it’s from a copyright free source/royalty free because there wouldn’t be any legal issues there.
And no, it wouldn’t necessarily be taken down unless it was reported or Episode was more thorough with reviewing, but if the original creators saw it/copyright holders - there would be trouble because you’re not legally allowed to use it/them (worst case scenario), if the original creators didn’t see and Episode realized you had content in your story you didn’t have the rights to use, you may simply just be asked to remove it from the story (in a better case scenario) while the story is pulled from the app until the copyrighted content is gone, but it’s best to avoid any of that from the beginning even if the likelihood of the original creators seeing it is low or minute. 