YouTube videos come with 2 types of subtitles.
1. Pre-Uploaded: Where the subtitles are uploaded manually by the video creator.
2. Auto-Generated: Subtitles that YouTube automatically generates for the video uploader.
Mostly, the content that comes out as un-formatted or without any punctuation is because the captions for that particular video are auto-generated by YouTube.
If you use a video with pre-uploaded captions you will get content with proper grammar as well as punctuation.
For the auto-generated captions, you can use the punctuation feature and paragraphing feature available above the post description box to get your content formated.
P.S. Not all videos with pre-uploaded captions will contain punctuation and formatting.