The games will run at their original resolution and framerate that they were designed for which for most games was 720p 30FPS.
However they will reach their framerate target much more often thanks to the increase in power. If a game targeted 60FPS but could only reach 40-45FPS most of the time, it will pretty much run at 60FPS on the One. Sadly dynamic resolution wasn't really a thing during 7th gen. Otherwise we probably could've seen them run at 1080p on the One (or at least on the X).
Speaking about X there are a couple of games that are enhanced for the Xbox One X.
Out of the ones you mentioned, Fallout 3, Portal 2, Red Dead Redemption and Halo 3 are X enhanced. However there's no need to get the 360 version of Halo games when you can just play Halo MCC.
Currently the BC team is working on the Series X but it's pretty much confirmed they will add more 360 (and original Xbox) games once the system has launched, so it's likely we get to see more enhanced 360 games.
