On: ENTP - Si subtype
I believe no matter what type you are, there are subtypes of that type based on what function out of all 8 functions is that person heavily investing in.
I believe the Si subtype of ENTP (which I believe I have met in real life at least once) is going to appear the most alike to a relatively balanced and not too serious or critical version of the ISTJ when directly focusing on Si for its own sake.
This is based on the extention of the predicament coming from socionics, where you either focus mostly on function slots 1, 4, 6, 7 or function slots 2, 3, 5 and 8.
This subtype would belong to the 1st group, focusing mostly on Ne, Si, Te and Fi. Being given an ENTP in their base type this will typically manifest as primarily being Ne and Te focused since they are of the NT temperament.
However there is I believe a given shift that takes place once the ENTP will shift from Si for the sake of Ne to pure Si focus for the sake of it.
To find out what exactly happens and why, I believe we have to take a look at the interface of preferred functions available here, thus Ne, Te and Fi.
This ENTP has chosen to prefer Si>Ne, thus Ne cannot be in their top 2 slots and thus shape their core temperament (NT, NF, ST, SF) so what we are left with is Fi and Te.
Based on our I believe common basic understanding on how functions work, it is natural to conclude the only function to be paired up with Si here is Te, since there is no type that has 2 introverted functions in their dominant amd auxiliary function slots and thus Fi is out of question.
And why ISTJ not ESTJ?
- because the primary focus has been stated to be Si not Te, which means that it cannot be a Te dominant type. If it were an ESTJ, the ENTP would choose Te as their primary focus - which is honestly not that uncommon in practice I believe, especially for this highly Ne and Te valuing type of ENTP.