I would prefer to just copy them - then you get to choose when updates get merged into each project which uses them.
Otherwise, you might find that changes to the common code break your dependent projects at inconvenient times...
Version control systems such as Git or SVN allow you to do this.
But, if you really do want to do it, you can add a file as a "link":

This takes you to:

Choose 'Advanced>>'
Then choose ‘Link to alternate location’, and enter the path to your source files:
A complex system that works is invariably found to have evolved from a simple system that worked.A complex system designed from scratch never works and cannot be patched up to make it work.