That's the great thing about it, use the bits you need and leave what doesn't work for you. I use the index sparingly, to make note of things that I might need to reference later but might be difficult to find. I usually do this at the end of the month before I store the book away. The future log and calendar are just there in case I need to write down a date and don't have my phone handy, because I, like you, use my phone's calendar because sometimes I need push notifications. If those thing are truly useless to you, leave them out. It took me several iterations to find the ideal setup for myself, I started out with all the things and dropped what I didn't use, went to a small book for ease of carry, now I am never without my little external hard drive. Check out the Little Coffee Fox site for a more artistic take on bullet journaling. If you don't see something that suits your needs, make it up!
My dream is to be able to create my ideal notebook myself (since I was making a ton of books around this time last year and so on). However....it takes a LOT of thinking about what you want to use the book for! I honestly have 4-5 notebooks "on-going" at the moment, but they are all dedicated to different purposes. Maybe I don't have a single notebook "Ideal"... :)
I've tried a lot of different systems myself. At work I use a combination of daily checklist that I created for myself and the monthly momentum planner worksheet from productive flourishing. https://www.productiveflourishing.com/free-planners/ with blank notebook paper for meeting notes. Work stuff stays in my office, so I keep that totally separate. I have a sketchbook and a small notebook that I use to keep notes about video games that I am playing. So no, sometimes "one notebook for every purpose" is not realistic. The little bullet journal is just my everyday carry thing, sometimes things that go into it get transferred elsewhere once I have access to the correct system.