One thing I’ve wondered is how broad is the autism umbrella? How much do two autistic people have in common other than being different than neurotypicals and thus having social difficulties? It seems to me that a condition defined by a difficulty with social signals could be extremely diverse in its causes and actual underlying conditions.

They might share a lot or might share nothing. Autism is an umbrella diagnosis over a wide range of behaviors and support needs. Diagnostically, it currently has a 1-3 scale across two broad support areas, with one being low support needs and 3 being high support needs. (From the DSM.)

That said, the reasons for autism are not well understood, and there may be many different root causes that lead to the umbrella diagnosis.

Additionally, experiences vary widely -- some autistic folks do not ever feel hungry and must be reminded to eat (or build support systems for it). Others do not ever feel full and must be reminded to not eat (or build support systems for it). Interoception (the perception of your body's internal signals) is impaired for many autistic folks.

Some of us experience misophonia, and get extreme fight or flight responses to audio signals -- personally, I get extremely agitated if I hear someone eating with their mouth open. (To be clear, it's not annoyance. It's fear/anger/hate/danger sense.)

Don't get too bogged down on the modern "autism" term. It's almost certainly going to change as we learn more about it.

Right now autism and neurodivergent cover huge swaths of behaviors/traits. This is especially true in the "social arena".

Just like back in the day when every kid that bounced their leg got an ADHD diagnosis. Now we know ADHD presents in a ton of ways and is usually present with other personality traits.