I know one is from birth as I've had it and one tends to be caused by environmental factors ie bullying which I've also had intensely.
I was your typical high functioning autistic child/teen. wanted to have friends but didn't quite have the social skills. despite a slow start I managed to have friendships and relationships in my earlier life.
However as I've got older I found I've got more cynical stuff has happened, people I thought were friends have left or stopped including me in their life anyway after many years of friendship. Also watching stuff that went on with family members who were in committed relationships for many years then their partner just cheats on them after over 20 yrs ....it just makes me think if you don't know someone after 20+ years when do you know them?
Physical restrictions, poor health and living out of town made me physically isolated too from being unable to get to social events regularly for many years and now I find I'm heading for 50 and my dreams are of living remotely, away from people, no longer missing human company (I prefer dogs - that could be the autism I've always related better to animals than people). I've no desire to work or do anything that involves 'too much interaction with too many people'.
Even to the extent that I'd love to adopt an other dog but am so convinced I'd be rejected as a suitable owner I can't start the process and it would also involve 'being social' (many rescues are now on FB and want to put photo's of the dogs in their new homes with their new owners - I don't do social media at all) and then want updates for the rest of the dogs entire life! Breeders are the same now they want to be part of the puppies life forever and I find that way too intrusive.
Long gone are the days you turned up at a rescue and just choose a dog and took it home and never had to deal with the people again. People think there must be something wrong with you if you aren't on social media cos everyone is.
Personally I'm inspired by a book I just read about a guy who lived 27 years in the woods without any human contact. There was also some debate about whether he had Asperger's or SPD.
I also spend alot of time daydreaming which I read recently is known as Maladaptive Daydreaming. This can be common in both autistic girls (I'm female) and SPD too.
Could I have both? or is it likely just cos of the autism cos I don't understand forgiveness well (for people I feel betrayed me) and taking things literally etc I have struggled to 'get over' when people have bullied me, let me down, surprised or shocked me?