Short answers, pulled from the story.
The National Football League formed in 1920 with a rule that every game counted toward the championship. Teams played each other in a league that had no preseason yet.
From 1970 through 1977, the season included fourteen regular games and six exhibition games. Team owners used their Sherman Anti-Trust Act exemption to expand the exhibition schedule significantly during this period.
On the 21st of July 2020, the league canceled all preseason games before training camp opened. Every team played one home and road game before the regular season started on the 10th of September 2020.
Dallas Cowboys and Los Angeles Rams played at Aloha Stadium in Hawaii on the 17th of August 2019. This marked the last NFL game held at that constant site before condemnation ended its use in 2020.
Coaches rarely start starters because teams schedule joint practices instead allowing first-team players against other first-teams in controlled environments. Rote plays dominate offense to avoid tipping hands to future opponents.