Short answers, pulled from the story.
NFL teams merged during World War II because so many players entered military service that individual franchises could not field full rosters. The Pittsburgh Steelers merged with the Philadelphia Eagles in 1943 to form the Phil-Pitt "Steagles," and the following year the Steelers merged with the Chicago Cardinals to form Card-Pitt for the 1944 season.
In 1966, Pete Rozelle promised Congress that NFL franchises would not move. The promise was made to secure passage of Public Law 89-800, which exempted the AFL-NFL merger from certain antitrust laws. Despite that assurance, several franchises relocated after the law passed.
Los Angeles went without an NFL team from 1995 to 2015, a span of twenty years. The St. Louis Rams received approval to return in January 2016, and the San Diego Chargers relocated to Los Angeles a year later.
When Art Modell announced in 1995 that he was moving the Cleveland Browns to Baltimore for the 1996 season, the NFL, the city of Cleveland, and Modell reached an agreement allowing the Browns franchise and history to remain in Cleveland, with a new Browns team promised by 1999. Modell received a new franchise made up of players from the 1995 Browns roster, which became the Baltimore Ravens.
The San Diego-Tijuana market is currently the largest U.S. metropolitan area without an NFL franchise, with over 3.2 million residents. The San Diego Chargers relocated to Los Angeles in 2017.
Only two of the 15 charter franchises that formed the league at the start of the 1920 APFA season remain in operation today. The other thirteen folded, merged, or were otherwise absorbed across the following decades.