Does the Florida Cession have anything to do with Adams-Onis Treaty?

Yes, the Florida Cession is directly related to the Adams-Onis Treaty.

The Adams-Onis Treaty, also known as the Transcontinental Treaty of 1819, was an agreement between the United States and Spain that resolved a boundary dispute between the two countries and resulted in the United States acquiring Florida from Spain. As part of the terms of the treaty, the United States paid Spain $5 million for Florida and relinquished its claims to Texas. The treaty was negotiated by U.S. Secretary of State John Quincy Adams and Spanish diplomat Luis de Onís, and its provisions also defined the boundary between the United States and the Spanish territories in the West, extending from the Gulf of Mexico to the Pacific Ocean. The Adams-Onis Treaty ultimately led to the annexation of Florida by the United States in 1821.