What are sites that curate a collection of well-done, but simple explainer videos and product demo videos?
Off to a good start:
You are looking particularly at curated explainer videos only or collections of curated content?
Either, really. My goal is simply to see some great explainer videos.
Also, Kurzgesagt is an awesome explainer video channel. https://www.youtube.com/user/Kurzgesagt/videos