What is the mechanism for combining prayers, and is it permissible to combine them due to visiting, or the arrival of guests, or studying?

If the mechanism of combining prayers refers to the method of combining them, then it is done by combining the Dhuhr and Asr prayers at the time of one of them (either by bringing them forward or delaying them), and similarly for Maghrib and Isha, while observing the correct order. Visiting, studying, and hosting a guest are not valid legal excuses for combining prayers. Rather, the valid legal excuses are travel, illness, and rain. Some scholars have also permitted combining prayers for any excuse that makes separating them difficult.