You cannot self-diagnose, and only a trained medical professional can diagnose autism. By trained medical professional, I mean an autism expert who is either a therapist (PHD, PsyD, LMFT, etc.) or a psychiatrist.
Most people who use terms like autistic, bi-polar, sociopath, narcist, etc. are using the terms incorrectly.
Also, I suspect you are correct that most ASD diagnoses do not use brain scans and rely on a trained professional's judgement and observations. That fact does not mean that autism does not exist or that some autistic people may have physical differences from neurotypical people.
Most people who use terms like autistic, bi-polar, sociopath, narcist, etc. are using the terms incorrectly.
Also, I suspect you are correct that most ASD diagnoses do not use brain scans and rely on a trained professional's judgement and observations. That fact does not mean that autism does not exist or that some autistic people may have physical differences from neurotypical people.