The name of this American city began as the name of a small chapel in Italy.
That chapel is the Portiuncula, a little stone church near Assisi. Its formal name was Santa Maria degli Angeli, "Saint Mary of the Angels," and its nickname, the "Little Portion," gives the Spanish Porciúncula. The Los Angeles Almanac calls it the Assisi chapel of Saint Francis.
On August 2, 1769, the Spanish expedition of Gaspar de Portolá reached the site. August 2 was the Franciscan feast of Our Lady of the Angels of the Portiuncula. The friar Juan Crespí named a river for the occasion. The Los Angeles Almanac gives the name as Nuestra Señora de los Ángeles de la Porciúncula, and La Nación adds la Reina to it.
The town came twelve years later. On September 4, 1781, 44 settlers founded a pueblo under the governor Felipe de Neve. They built it near the Tongva village of Yaanga, on the river Crespí had named. Wikipedia gives the founders' name as "El Pueblo de Nuestra Señora la Reina de los Ángeles." It translates as "The Town of Our Lady the Queen of the Angels."
The exact original wording is disputed. The historian Doyce Nunis argued for "El Pueblo de la Reyna de los Angeles," while Monsignor Francis Weber favored a name closer to the river's. What is clear is that the name honored Mary, not the landscape.
Over time it was shortened. California became part of Mexico after its independence in 1821, and the United States took it in 1848. By the time Los Angeles was incorporated as a city in 1850, the long title had shrunk to the last two words. La Nación reports that "Los Ángeles" became the usual form in official documents, maps and everyday speech.
The English pronunciation then drifted too. Wikipedia says the modern anglicized form has been the most common since the 1930s, and the U.S. Board on Geographic Names made it the federal standard in 1934. A respelling pushed by the Los Angeles Times in the early 1900s to match Spanish did not catch on.
The Spanish plural, Los Ángeles, means simply "the angels."