I see you tagged construction in your post but I believe this can be applied in any field/industry. The position I am currently in, wasn't my first position. When I first started in my career, I worked for a retail store, then a restaurant. From the restaurant, I was able to meet business professionals which created an opportunity for me to work in an office environment as a receptionist. I was unsure of what I wanted to do honestly but I explored different opportunities as they presented themselves.
From the work experience, I was able to eliminate what I knew I wasn't interested in and through developing relationships with other professionals, I was able to ask questions and receive guidance on the jobs they had in their past.
It was in my early twenties, while working as a receptionist that I volunteered to assist the Human Resources team where I worked. While assisting the HR team, I learned what I really enjoyed doing. As I grew in my skills and knowledge of what I knew I enjoyed, I was able to tailor what I wanted around my experience.
As you can see, I didn't know exactly what I wanted to do out of high school or while even in college. It was easier for me to know what I wasn't interested in. As you grow, there isn't always a defined answer as to why individuals choose their specific positions. For those that do know exactly why, that's absolutely wonderful as they've been able to hone in to exactly what they want to do. In particular, I enjoy learning new things and gaining knowledge in areas where I think I'd be interested.
I hope I was able to answer your question sufficiently. Good luck in whatever career you are thinking about.