The physical versions of Call of Duty: Infinite Warfare will require the Infinite Warfare disc to be inserted into the console tray to play Modern Warfare Remastered.
The PlayStation 4 version is a “full game digital download” redeemed through a voucher code inside the box. The Xbox One version contains Modern Warfare Remastered on the disc, but requires an additional download. Regardless of platform, an internet connection is necessary to play Modern Warfare Remastered.
For Steam users, “A Steam key will be provided in the Legacy Edition of Infinite Warfare that will provide access to both Infinite Warfare and Modern Warfare Remastered.” The FAQ does not clarify if the games can be run separately or through one installer.
The remastered version of Call of Duty 4: Modern Warfare is only available in the Legacy, Legacy Pro, Digital Legacy, and Digital Deluxe editions. The game cannot be bought separately, and requiring the Infinite Warfare disc to run stops console players from buying one of the special editions, using the Modern Warfare Remastered code, and then selling the rest or trading it in for store credit.