What does winning entry mean?

[v0031]

On September 18，1793, George Washington laid the cornerstone to the United States Capitol building, the home of the legislative branch of American government. The building would take nearly a century to complete, as architects came and went, the British set fire to it and it was called into use during the Civil War.
Today, the Capitol building is part of the Capitol Complex, which includes six Congressional office buildings and three Library of Congress buildings, all developed in the 19th and 20th centuries.
As a young nation, the United States had no permanent capital, and Congress met in eight different cities, including Baltimore, New York and Philadelphia, before 1791. In 1790, Congress passed the Residence Act, which gave President Washington the power to select a permanent home for the federal government. The following year, he chose what would become the District of Columbia from land provided by Maryland.
Washington picked three commissioners to oversee the capital city’s development and they in turn chose French engineer Pierre Charles L’Enfant to come up with the design. However, L’Enfant clashed with the commissioners and was fired in 1792.
A design competition was then held, with a Scotsman named William Thornton submitting the winning entry for the Capitol building.
In September 1793, Washington laid the Capitol’s cornerstone and the lengthy construction process, which would involve a line of project managers and architects, got under way.


Did he submit the work and then won or he submitted the work which has been awarded already?

[Bacon King]

He submitted the work that (after he submitted it) won the competition

[muon2]

In a competition the adjective winning is used to describe the competitor or action that won the competition. For example I could say France won the 2018 World Cup or France was the winning team in the 2018 World Cup. Similarly I could say that Yuki Kawauchi had the winning time in the 2018 Boston Marathon.

A contestant in a competition is said to have entered the competition, particularly when the competition is open as opposed to a league. That makes the contestant an entry in the competition, especially when the contestant is inanimate. In the quoted piece the contest was between plans for the Capitol, so those each plan was an entry in the competition.