I think this may be a very common question (but not for me). Can someone please tell me how did the text of Kaddish first compiled?
The concept of Kaddish is mentioned in numerous places in the Gemara (See Berachos 3a, 57a, Shabbos 119b, Sukka 39a, Sotah 49a).
The exact authorship of Kaddish is unclear.
The Rokeach (Pirush Sidur HaTefilla) attributes it to the Anshei Knesses Hagedolah (Men of the Great Assembly) , who authored the first standardized text of Tefillah (see Berachos 33a). This is also the opinion of the Aruch Hashulchan (55-1)
הקדיש הוא שבח גדול ונורא שתקנו אנשי כנסת הגדולה אחרי חורבן בית ראשון, והיא תפלה על חילול שמו יתברך מחורבן בית המקדש וחורבן ארץ הקודש ופיזור ישראל בארבע כנפות הארץ, ואנו מתפללים שיתגדל ויתקדש שמו יתברך כמו שאמר הנביא 'והתגדלתי והתקדשתי ונודעתי לעיני גוים רבים'
However the Orchos Chaim (Laws of Kaddish, 1) writes that the Aramaic text of Kaddish that we have was composed during the times of the Tanaim. (Tosfos (Berachos 3a) may support this - Tosfos explains that the reason Kaddish is in Aramaic is because there were laymen whose main language was Aramaic, who didn't speak Hebrew. My history isn't great, but I don't think Jews during the time of the Ansei Knesses Hagedola spoke Aramaic as a primary language.)
This summary brings up the following on page 6
Concerning the origin of Kaddish that you asked, we can not trace the practice to a clear and definitive source found among our ancestors’ writings. However, those who followed them based their recitation of Kaddish upon this verse (Leviticus 22, 32) And I will be sanctified within the people of Israel and upon what our Rabbinic leaders extrapulated from the verse: that any prayer which results in the sanctification of the name of G-d must be recited only in a group of ten men.
From theses teachings we can conclude that when ten men congregate to perform a mitzvah, whether it be for prayer or for the study of Torah, they must recite a prayer in which they sanctify the name of G-d. As a result, at the conclusion of the first section of the morning prayer, Pseukei D’Zimra, after reciting the Bracha in Yishtabach, those congregated should recite Kaddish because they have the completed the mitzvah of reciting Pseukei D’Zimra and they are about to perform an additional Mitzvah, the recital of Kriyat Shma and its blessings before and after . . .
And they should recite Kaddish after Shmona Esrei because it too is an independent Mitzvah and it is not connected with that which is recited after it. And another Kaddish should be recited after the Torah Reading because in order to read from the Torah, ten men must congregate together. And another Kaddish after reciting the Kedusha in OO”Vah L’Tzion because it too is a Mitzvah unto itself and it is a Prayer of Sanctification of G-d’s name and requires the presence of ten men.