Hard to answer this in just a forum post, we are talking about 7 years of development here and as @wielkiciensteam mentions the people doing so have shifted, and with them the craft likely shifted a bit as well. 
I have written a few dev diaries in the past about how I approached research during the period when I worked on EU4 (2014-2018 more or less). This is one of them: https://forum.paradoxplaza.com/forum/threads/eu4-development-diary-2nd-of-may-2017.1018557/
We have in the past given interviews related to how we approached EU4, some of which resulted in this: https://helda.helsinki.fi/handle/10138/315919
Me and @Arheo also talked a bit about it (IIRC) here: https://podhistory.libsyn.com/ep-44-playing-with-paradox-games
If you have questions I would encourage you to get in touch by email and we will see if we can help
I have written a few dev diaries in the past about how I approached research during the period when I worked on EU4 (2014-2018 more or less). This is one of them: https://forum.paradoxplaza.com/forum/threads/eu4-development-diary-2nd-of-may-2017.1018557/
We have in the past given interviews related to how we approached EU4, some of which resulted in this: https://helda.helsinki.fi/handle/10138/315919
Me and @Arheo also talked a bit about it (IIRC) here: https://podhistory.libsyn.com/ep-44-playing-with-paradox-games
If you have questions I would encourage you to get in touch by email and we will see if we can help
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