I've been both the step child and the step parent ..and can say, it is Never that black and white. And it's stupidly reductive to say it is.
As a step child on both sides, my mother's second husband treated me as his own, not so much my elder siblings because they were older and had a good relationship with our bio dad. But then as soon as he and my mom split up, and I said I didn't want to move 100 miles away he ditched me. After 13 years of being an excellent parent to me.
My step mother was very standoffish but I have no relationship with my father anyway so that's expected. She gets on well with my siblings, and they would say they never view her as a parental figure.
As a step mom, I was devoted to my stepson. Adored him. I was his primary care giver. I did everything for him that his parents couldn't be bothered to, I took him to his first day of school, all his medical appointments, all his parents evenings. He was mine. But then I split with his dad and despite having had a child with his dad too, I never got to see him again.
All of those were hard and difficult, and all required me to find parts of myself I didn't know I'd have to.
It's never that simple.