Is it normal for toddlers to be really hungry one day and not want to eat barely anything the next day?

Yea. I always tell my patients to see what they eat in a week rather tha day to day.
Yes. This is very normal behavior for a lot of toddlers. They can be picky and barely eat anything one day, then be ravenous the next. As long as you are providing nutritious meals to your child and their growth and development is within the normal range, dont worry too much about it as it is usually a phase. If his diet is too restricted or there are concerns about his growth then see your doctor.
Yes. Toddlers eat based more on hormone/activity related needs rather than habit. Over time their body usually decides how much and what they eat. As we age, our habits and cravings make us less responsive to internal ques, and we end up over or under eating. It would be nice if we could avoid habit eating. What you report is normal.