To add these to your Blender scenes use File>Append. Select the "Pistons.Blend" and you can either append entire collections on pistons or individual pistons by choose objects. If you append individual pistons be sure to get the child "b" part as well.

To learn how to attach pistons dynamically in to your creations check out this tutorial where I show how to use constraints to tig your models without complicated rigging and bones.

https://youtu.be/Zd0nIlfRgsU