You can get really into the jungian-esque theory too where the four-letter typing is just a shorthand for which of the eight"cognitive functions" you tend to use most often. For instance an INTP uses most:

1. Introverted Thinking
2. Extraverted iNtuition
3. Introverted Sensing
4. Extraverted feeling and then in times of stress/distress you may use the less comfortable shadow functions.

It's still just based on a woo-woo theory, so basically vibes, but I found this level of mbti way more interesting as a way to talk about thoughts and behaviors and what individual people tend to default to in which kinds of situations.

In introverted situations you might tend to use your own internal logic to make important decisions, priorities, and do important work, and then for fun and flavor use memories and details to inform yourself. In extraverted situations you may tend to lead with free association and connection of ideas and people. And might tune into the emotional expression of others as a lesser but still important extent

Maybe it rings true maybe it doesn't. But according to this framework, an INFP is very different, way more than just 1 letter different.

1. Introverted Feeling
2. Extraverted iNtuition
3. Introverted Sensing
4. Extraverted Thinking

This person uses their internal feeling about things to make decisions and lead in introverted settings. Their feeling are literally the most important thing they lead with.

And the extraverted thinking at the end means that this person will tend to want to make and use external systems to organize things, but not necessarily be that sophisticated in this.

The fact that these two "types" show up so differently in the theory is the biggest flaw in the tests, and the theories. If the test shows that you're 50% F and T, they would say that you have to read both descriptions and pick the one you vibe with best, because you can't be in between.

(On the other hand, it's super chill to be 50% E/I because those two types are basically the same, just more or less introverted.)

Consider ENFP is

1. Extraverted iNtuition
2. Introverted Feeling
3. Extraverted Thinking
4. Introverted Sensing

They have all 4 cognitive functions in common with the INFP, just favoring the extroverted functions. So it's way easier to imagine being on a spectrum of ENFP/INFP depending on how introverted or extroverted you are.

How well does this actually explain or predict human behavior? Who can say. But it's fun. No way it should ever be used in a professional/work setting imo.

Source: I listened to way too many podcasts and YT videos in the 2010s about this stuff.