All questions about writing a book

What do like/want to write about?
I love writing mysteries and tapping into many fantasy and supernatural genres. I probably like writing urban fantasy the most. One day I hope to write a multi POV mystery novel (don't know if I want it to be fantastical or not.)

For me it's when they spoil their own story what I mean by that is when at the end of a scene or a chapter they'll say something like "if only I knew that those were the last good days of my life." or "I wish I knew then so maybe I could have saved him/her." or the worst for me anyways is when the author will put something like "One of them will die." LIKE WHY SPOIL YOUR OWN STORY I know it's meant to be suspenseful but at the same time it makes me so mad it's not as common anymore but I do still see it sometimes but I think it's just as bad when the writer does do that especially the "one of them will die" and then the character dies but oh wait they're not actually dead so you did that for what??

I am currently planning out a novel to write that I will eventually publish when it is completely written and edited. I have an idea of my story but I would love some opinions or recommendations that could help me.

Could some of you give me things:
- you would love to see in a novel
- you would hate to see in a novel
- any themes you love
- any love interests etc.

I encourage a discussion that could help me on may journey!
Thank you

Choose a video from this playlist and I’ll give you feedback, my opinion, a rating, or something—I dunno, on it. It can be any video from it.

https://youtube.com/playlist?list=PLZEqxA_4nN4kkmUizanKWEWN6YWCetOO2

I always have stories I want to write. The beginning and the ending are already established but I'm always having a hard time to write the main body of a story. Any tips?