What is the ruling on odors resembling the smell of feces that exit from the anus after relieving oneself, and is purification from them and changing underwear obligatory, and what is the ruling on slight moisture that exits with air?

There is no need for istinja’ (washing after defecation) or changing clothes due to the smell of feces, as the smell itself is not impure (najis), and acquiring it from impurities does not cause harm. If you find moisture and doubt its purity, consider it pure, because the default state is purity until certainty of impurity is achieved. If you doubt whether the wetness is sweat or something that exited from the anus, the default is that nothing exited from the anus. Wudu’ (ablution) is only required upon hearing a sound or perceiving a smell. It is advised to disregard obsessive thoughts (waswasa).