Full answer
A basic digital antenna ($20–$40) picks up CBS, Fox, and NBC broadcasts over the air in most metro areas. These three networks carry the majority of regular-season NFL games including Sunday afternoon matchups and Sunday Night Football.
NFL+ offers a free 7-day trial that includes live local and primetime games on mobile devices. After the trial it costs $7.99/month, but the trial alone covers a full week of games.
Tubi and Pluto TV occasionally carry NFL-licensed content and some out-of-market replays for free. Yahoo Sports streams local-market games free on mobile with no login required during the regular season.
Library cards in many cities unlock free access to streaming services like Kanopy, and some public libraries offer free Paramount+ trials — which carries Thursday Night Football in select markets.