I think the league assigns the games where teams have not made arrangements themselves.
Patriots have played the Giants in the preseason since like 2000? I think, and have played the Giants as the last game of the preseason since 2003 (this is the first year it's a different team (the Raiders....where McDaniels is)).
This Wiki entry mentions a bunch of frequent match ups of other teams: https://en.wikipedia.org/wiki/NFL_preseason