I don't think that they needed to invest much into it, honestly. Xbox One models had AMD CPUs and AMD GPUs. Xbox Series X & S have AMD CPUs and AMD GPUs. They probably had to do very little to make the older games run on the new consoles and the more powerful hardware naturally improved the performance. It's basically a PC, and improving the performance of games by upgrading the hardware is what has always been part of the fun with PCs. It's kind of a new thing for console gamers only because, in the past, console manufacturers tended to change their architecture too much between generations. It's not just Microsoft that has kept things pretty similar between generations this time, either. The PS4 and PS5 use AMD CPUs and GPUs, as well, which is why the PS5 will offer better performance for PS4 games, too.