How do I make and reuse parts?

It may be implicit somewhere, but I found no explicit mention or example showing how to make a "thing" in a file, then reuse it both in that file and in other files.

For example, a library of standard bolts and nuts, screws, nails, etc. Or a parametric rafter for making a roof on a house. And so on.

If this capability exists, please expose it more prominently.