Types of Governments - Monarchy

Monarchy
 A monarchy is a form of government in which sovereignty is actually or nominally embodied in a single individual. Forms of monarchy differ widely based on the level of legal autonomy the monarch holds in governance, the method of selection of the monarch, and any predetermined limits on the length of their tenure.

 In hereditary monarchies, the office is passed through inheritance within a family group, whereas elective monarchies use some system of voting. Each of these has variations. In some elected monarchies, only those of certain pedigrees are eligible, whereas many hereditary monarchies impose requirements regarding the religion, age, gender, mental capacity, and other factors.

 Occasionally this might create a situations of rival claimants whose legitimacy is subject to effective election. Finally, there have been cases where the term of a monarch’s reign is either fixed in years or continues until certain goals are achieved.

 Monarchy was the most common form of government until the 19th century, but it is no longer universal. All European monarchies are constitutional ones, with the exception of the Vatican City, but sovereigns in the smaller states exercise greater political influence than in the larger.

It is surprising that there are many countries that abolished monarchy. So I want to know the differences between the countries that abolished monarchy and the countries has maintained a monarchy yet.

United Kingdom is a monarchy. I want to know how the United Kingdom is maintaining the monarchy.