Si vs Ni comparison (MBTI) - ISFJ or INFJ?

Here I explain the key differences (and some similarities) between Si (introverted sensing) and Ni (introverted intuition) according to MBTI. The ISFJs and ISTJs have Si as their dominant function, and INFJs and INTJs have Ni as their dominant function.
Keywords: MBTI, personality type, 16personalities, Myers-Briggs, Carl Jung, infj, intj, isfj, istj, cognitive functions