As the course creator you retain copyright to all of your content uploaded to Thinkific, but how about if you're looking to use someone else's content (e.g. a video from YouTube) within your course?
The best policy for this is always to obtain permission from the original creator of the content - they'll be able to let you know if they're OK with you using their content and how they'd like to be credited.
So provided that you first, gain the author's permission, and second, credit them somewhere within your course (according to their wishes), you should be fine.
(Please note that copyright of your Thinkific site and courses is attributed to you as the site owner, and so we are unable to provide assistance with any disputes around content ownership - i.e. this is friendly advice rather than legal advice!)
do site clients retain ownership rights to content?
Absolutely - we don't make any claims on your content, the copyright stays with you :)
If you preview your site you'll see the copyright line at the bottom of the page - it's based on your Site Name, so you can head to the Basic Settings page to change that if you need to.
Happy to help if you have any further questions at all.