The National Autistic Society - autism.org.uk - is a really useful source of information.
Briefly, and very roughly, autism is often described as a spectrum because people can have a diagnosis of autism but have a wide range of ability/intelligence. There are people with autism with severe learning difficulties/disabilities but also people with autism who are highly intelligent, able and talented plus all the stages in between.
However, even the most able will likely have some difficulties in life and education, often around friendships, imaginative tasks, understanding things which are not explained clearly or based on logic and facts. People with autism often like routine and things to happen as expected and can become very anxious when outside their comfort zones. They can also struggle with noisy and busy environments. These are all generalisations but just to give you a bit of a picture.
So, although, a child may be perfectly capable of coping with the academic demands of mainstream school, and sometimes may excel academically, they may still need support with some aspects of school life.
You apply for schools just like everyone else. I doubt your son's needs will over-ride catchment area rules, though if you find a school that you think is particularly strong on supporting students with autism, you could try making a case for him. And you should discuss an application for an EHCP with all the schools you visit. Those who are supportive of an application should go at the top of your shortlist.