A lot of components and assemblies from China are available on ebay, banggood, and other sites at very low cost. However, documentation and usage instructions are often unavailable or limited in quality (at least in English).
Here, I'm documenting my experiences with some things in the hope it will be useful for others.
First, a caution: we're talking very low cost units here (a few dollars or less), so quality and reliability are often not the greatest. So we're aiming at experimenters and hobbyists here, where malfunctions and failures are not a serious problem. If reliability and safety are issues for you, you're on the wrong page here.
Also, there are many vendors and designs, so things that look similar may not be exactly the same. What I'm discussing here are the specific ones I've looked at in some detail. YMMV.