Hallowe'en is very much an Irish thing as well as Scottish. As a child in the 70s we used to create papier mache masks in art class in school, and make or put together a costume - I had a witch mask most years, and a succession of black bin bag costumes, along with a hat bought somewhere. We used to traipsing around the neighbourhood knocking on doors for apples and nuts. We would end up with very bruised apples and lots of peanuts, walnuts, almonds, etc. Dad would crack open the nuts when we got home, and mum would take the apples and eventually make stewed apples, apple crumble, or apple fool.
Then there was a scare in the US about apples with razor blades inserted in them by psychos who hated children, people became more aware of food allergies too, and people were urged to only give or keep wrapped treats. Nuts and ibv peanuts fell out of fashion and apples and anything home made got discarded. Small size wrapped sweets were ideal as you knew what was in them, and you knew they hadn't been tampered with.
The economy of Europe in general grew, and manufacturing in Asia took off, meaning people had money to spend on festivals.like Hallowe'en, buy cheap (almost disposable) costumes, and make a big day of it. At the same time, movies with Hallowe'en and American TV shows presented an attractive vision of the possibilities of the holiday. Trick or treating crossed the Atlantic, along with other holiday ideas like decorating your house. Trick or treating is the same basic idea as the time honoured traditions of Ireland and Scotland.
OP, I do feel that some of your questions and puzzlement spring from autism. In fact, I wondered if you were a friend of mine (diagnosed with autism in her 30s) who feels the same way about Hallowe'en and other festivals.
Hallowe'en was traditionally a time between seasons at the end of the warm and light filled part of the year, when darkness and cold descended on the northern hemisphere, and the dormancy and scarcity of winter lay ahead. In ancient Irish (and possibly Scottish) tradition, it was a time when the veil between life and death became 'thin' and people remembered their ancestors. It was always a time when people considered the cycles of life and death. Traditions of spirits wandering and visiting their old homes led to disguises, wandering from house to house, and offerings of food, with agricultural products like turnips carved to represent the spirits of the dead. There were traditions involving games of divination too. The feast of Samhain also included the lighting of fires, both in individual hones and as a community event. When I was a child the neighbours used to gather for a big community bonfire in a local park after we were all finished gathering our Hallowe'en booth. A hilly peninsula a few miles away was the site of a massive bonfire every year (still is). There has been a revival of pre-Christian Hallowe'en festivities in Ireland recently, which has taken place alongside the Advent of trick or treating, pumpkins, and other innovations.
The Christian element of the season is the feast of All Saints and the feast of All Souls. As a family back in the day, and even now, we always combined the traditional and the RC All Hallows celebrations. All Saints Day is usually a holy day of obligation (to attend Mass). One of my DDs had a friend (in the US) whose devout Baptist mother shunned Hallowe'en because she believed it was a celebration of Satan's birthday, but the focus on the dead and the global fascination with cycles of life and death are ancient and universal (see Dia de Los Muertos and various other ancient feasts all around the world), that Christianity saw as an ideal instinct to use as a means of teaching about salvation and the promise of eternal life.
All the stuff about ouija boards and dabbling in the occult, and many of the basic premises of horror movies are not specifically related to Hallowe'en at all. I personally would never engage with any of that, or tarot readings, etc. But I have no qualms about the usual Hallowe'en festivities.