Self studying an AP subject is a common route in Turkey, because most schools do not teach AP. The question worth asking is not whether it is possible but in which subject it is possible.
The criterion
Whether a subject can be self studied depends on where the exam awards its marks. Where answers are simply right or wrong, a student can check their own work.
Where answers are read against a rubric, they cannot, because they cannot see whether their own answer would have earned the mark. That is the whole difference.
Subjects that suit it
Microeconomics, Macroeconomics, Psychology and Statistics are relatively suited: the syllabus is bounded, resources are plentiful, and most questions can be self checked.
Calculus also suits it, because whether a problem was solved correctly is not ambiguous.
Where it hits a wall
Subjects asking for experiment interpretation (Chemistry, Biology, Physics) and subjects asking for extended writing (History, Literature) are where the most marks are lost in self study.
The reason is feedback, not content: the student cannot see why an answer fell short of full marks and repeats the same fault for months. On its own this can account for a whole score band.
How it appears in the file
There is no difference at all between a score obtained externally and one obtained after taking the course at school; the certificate is identical and carries no marker.
The difference sits elsewhere: a student who took the course at school has it on their transcript, an external candidate does not. Some universities read course load from the transcript.
A realistic method
The syllabus is taken from the official page, past exams are worked through, and every free response answer is read against the rubric. Reading the rubric is the most critical and most skipped step in self study.
Where a student cannot judge their own answer against a rubric, this route does not deliver without a source of feedback.