I would like to briefly introduce the recently released HAELE 3D Hand Poses Pro, which I found to be very useful as a reference for drawing hands.

Windows only, requires a graphics card.

Hand Poses Pro is an app that lets you observe a very realistic 3D hand model in 360 degrees in any pose you like.

The difference with Clip Studio's 3D models is that the metacarpal bones in the palm are incorporated into the model as bones, so the way the hand spreads when opened and the roundness of the fist when clenched are realistically reproduced.

A variety of presets are available, including scenes where the character is holding a sword with both hands and scenes where the character is shaking hands.

I learned a lot from sketching poses created with Hand Poses Pro in Clip Studio, so I've explained it in detail in the video below.