Here is a list of the richest countries

Luxembourg (GDP per capita: $119,719)

Norway (GDP per capita: $86,362)

Switzerland (GDP per capita: $83,832)

Ireland (GDP per capita: $81,477)

Iceland (GDP per capita: $78,181)

Qatar (GDP per capita: $65,062)

The United States of America (GDP per capita: $64,906)

Denmark (GDP per capita: $63,434)

Singapore (GDP per capita: $62,690)

Australia (GDP per capita: $58,824)

so I know the USA is the richest country but this is the richest countries by GDP.

You should use GDP per capita when comparing countries GDPs

Germany is the richest in terms of GDP (nominal and PPP) , Liechtenstein in terms of GDP per capita

US$20,400 (nominal exchange) or US$29,700 (PPP). For comparision purposes, Mexico's GDP per capita is US$10,200 in nominal terms or US$14,800 in PPP terms.

Singapore is a developed country.

Singapore is the fourth wealthiest country in the world in terms of GDP (PPP) per capita, and the twentieth wealthiest in terms of GDP (nominal) per capita.

Singapore is one of the most developed countries in the world.