There is a huge difference between shyness and autism.
Shyness is something that people can overcome or grow out of, while autism is permanent and cannot be grown out of.
Shyness is not a disorder, but a state of mind. It‘s usually connected with low self-esteem. The person fears being judged by others, so they withdraw themselves. Autism on the other hand is a complex developmental disorder. It causes lack of social skills way beyond shyness, along with several other kinds of symptoms. People with autism struggle to understand non-verbal communication like body language, facial expressions and such. With up to 80% of communication being non-verbal and autistic people being pretty much „blind“ to nearly 80% of what other people communicate, it‘s hard to keep up with others and understand what they are communicating, which results in confusion, misunderstandings and such. People with autism cannot pick up social cues and are not aware of unwritten rules unless they are told about them specifically and they struggle to understand and follow various complex social rules. People with autism also have speech delay, which makes it hard for them to express themselves and communicate.
While shy people and people with autism may both be withdrawn in social situations, the reasons for it are very different. Shy people have pretty normal social skills and communication skills, but are afraid to use them for fear of being judged or making a fool out of themselves, so they may not socially interact very much and may feel nervous around people they don‘t know well. They can interact perfectly normally with people who they know better. People with autism don‘t socially interact much or well because they don‘t know how to do it and can‘t keep up with people whose social skills are unaffected and their social skills are not any better around people who they know than around strangers.
In addition to this, autism has a lot of other traits/symptoms, such as obsessive interests, strong need for routines, difficulty handling changes or surprises, sensory issues, motor skills issues and many more.
People with autism are wired differently than people without autism (neurotypicals). Their brain works differently. That is not the case with shy people. People with autism can be shy, but not all of them are. Likewise, neurotypical people can be shy, but not all of them are either. Shyness and autism have nothing to do with each other.
I have a mild form of autism called Asperger's syndrome.