Why does Australia have a small Union Jack on its flag?

Because they were all once British colonies and are still Commonwealth countries. If you have the opportunity to see maps of various countries in the 19th century, you will find that in fact, the national flag is set in the upper left corner of the colonial flag (it cannot be called a national flag).

It can be called a fashion or a practice at that time. Not only the United Kingdom, but also other Western powers such as France, the total number may be dozens.

Today, only a few Commonwealth countries still retain this tradition. This is because of the former colonial system, only the Commonwealth still exists, and it is only in name. Commonwealth countries also have governors, which have become

Symbolic positions, these countries are currently more connected economically, as well as the Commonwealth Games.

In fact, there are still some countries that have de facto colonies, but they are not expressed in a big way through a system like the Commonwealth, such as French Guiana in France. The United States also owns many islands in the Pacific, such as Guam.