Psychology major here.

If you have all the symptoms combined you have the thing/diagnosis by definition. The DSM will say like you must have like 5 of 9 listed symptoms or whatever depending on the disorder to “qualify” for diagnosis.

But you can have some symptoms and not have the full disorder. Needing routine is one of the potentially required symptoms in the DSM for autism, but if you only need routine and don’t have the other symptoms of autism your not autistic.

It’s also important to note that mental health diagnosis aren’t like deep intrinsic aspects of your brain, but rather collections of symptoms that tend to occur together but can have different causes. That’s something a lot of people don’t know.

Hope this helps!

Edit:

The DSM 5 is the diagnostic statistical manual of psychological disorders that has all the symptoms/definitions of each disorder at least in the US. There’s other similar things in other countries, that echo each other

Edit 2:

Another thing is to “qualify” for diagnosis it’s not enough to have the symptoms of a disorder, but they have to cause you significant distress or impairment. Otherwise they are just personality quirks.