Hiroya Oku likely starts with a concept or story idea. Then, he sketches out the characters and plotlines. After that, he refines the details and adds depth to the story through continuous revisions.
Well, smut manga typically features a lot of sexual imagery and scenarios. Regular manga, on the other hand, can be about anything from superheroes to school life and doesn't focus mainly on sexual themes. The main difference is the emphasis on sexuality in smut manga.
One example could be 'Love Hina'. It has a great story about a young man and his relationships with various female characters in a boarding house. There are some romantic and comical situations that border on the more 'smutty' side at times. The character development and the overall plot of the protagonist trying to find his place and love make it engaging.
The plot itself should be engaging. It can't just be a series of sexual scenes. Take 'Nana' for instance. It has a story about two girls named Nana who are on different paths in life. There are some romantic and passionate relationships in it. The overall plot about their friendship, career aspirations, and love lives holds the story together, and the 'smutty' elements are just part of the bigger picture of the characters' lives.
Well, without having read the 'takarakuji de 40 oku atattandakedo light novel', it's hard to say for certain. However, given the title, it seems likely that the story is centered around a situation where someone has come into possession of 40 oku, which could be a very large amount of money or some valuable asset in the context of the story. It might explore themes such as the impact of sudden wealth on one's life, relationships, and personal values. The 'takarakuji' part might imply that this wealth was obtained through a lottery - like event, and the light novel could then detail the various adventures, challenges, and emotions that follow this major change in the character's circumstances.