The related concepts are generally drawn from the individual scope and sequence documents. Of course the documents you use could also be the national curriculum documents of a particular country. You will also find related concepts in the descriptors of each PYP transdisciplinary theme and in the PYP strands in each subject scope and sequence. Drawing concepts from the different subjects, and putting them together into a central idea, will ensure that the unit is truly transdisciplinary.
This resource contains six tables, one for each transdisciplinary theme, broken up into strands, each of which contain several related concepts. Toddle has done a brilliant job, as always, of publishing these in an easy to use format.
I hope this resource is useful to you as you design your PYP units.