Saurus: The reason you want to “upload a youtube video” is that every time you do upload a youtube video to youtube, it will automatically create a post about your video on your blog. Then you don’t need to copy and paste, which might be a simple process but it’s actually easier to just upload a video and have your blog post created automatically. Also, it will save you from unintentionally forgetting to post your video on your blog. And if you have a lot of videos, it could also save you some problems.
Once you have your API Key, everything’s easy! No more annoying cut and paste of embed tags.
However, getting a WordPress API Key is not so straightforward if you host your own blog. To get a key, you need to sign up for an account at WordPress (choose the option of signing up for an account without starting a new blog). Then get the API Key by logging into your WordPress
And the API URL is: https://your.blog.address/xmlrpc.php
(More info at https://en.forums.wordpress.com/topic/api-endpoint-url)