What is the ruling on one who reaches puberty, or embraces Islam, or becomes clean from menstruation, or recovers from insanity, or returns from a journey, or recovers from an illness during the day in Ramadan; is it obligatory for him to refrain from eating and make up for the fast?

Those mentioned in the question fall into two groups:

The first group: The boy who reaches puberty, the disbeliever who embraces Islam, and the insane person who recovers sanity. These individuals must abstain (from food and drink) but are not required to make up (the fasts), as their accountability for religious duties is established at that point. The second group: The menstruating woman who becomes pure, the traveler who settles, and the sick person who recovers. These individuals are not required to abstain, and their abstention would not benefit them in any way; they are, however, required to make up (the fasts).

The difference between the two groups is that in the first group, the condition for accountability was met, whereas the second group was addressed with the obligation of fasting but had an excuse that permitted them to break the fast. Consequently, the sanctity of the day was lifted for them, and they must make up the fasts after Ramadan.