There is an Atrtscroll Sepher in English on topic of Kaddish which you can procure.
This elongated Kaddish which you are inquiring about is said at funeral & Siyum, both representing the conclusion of a chapter of life, be it someone who just passed away, or the life of a specific tractate. Hence the 1st part of Kaddish being the same for these 2 occasions, albeit on no Tachnun days such as today Yom ha’Atzma’ut this elongated Kaddish is relegated to a regular Kaddish, whereas by a Siyum this elongated version is still recited.