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.
I built https://www.speckbit.com where you can curate the best content around topics. There's interesting content around a lot of important things. Not exclusively explainer videos though.