What does the dot mean in English names, such as Sergeant Thomas mcpherson? What is SGT?

Sgt. is the abbreviation of title. Sergeant.-Sergeant/Sergeant

Some names with more than one capital letter are also correct.

For example: john mcdonagh, leann rimes.

Usually starts with "Mc" or "Mac"

Many China people's names are capitalized when translated into English. For example: Wang Zhenghua.