Understanding Halo 5's legendary ending?
At the end of Halo 5, the UNSC Infinity begins doing random jumps in order to escape Cortana. This parallels the beginning of Halo 1, where the Pillar of Autumn performs a random jump in order to escape covenant on Reach.
In Halo 1, the random jump leads the Pillar to discover the first Halo Ring. It's very likely that the parallel between Halo 1 and Halo 5 of "random jumps" to escape a threat has led the Infinity to discover a new Halo ring in the same way that the Autumn did.
In any case, the point of the legendary ending is that there are more Halo installations than were previously realized.
The Halo Rings with known locations prior to Halo 5 are Installation 00 (the Ark), Installation 03 (where the Composer is found in Halo 4), Installation 04 (Halo 1 & a replacement in Halo 3 that is also destroyed), Installation 05 (Halo 2 aka Delta Halo), and Installation 07 (quarantined by UNSC and Sanghelios forces in Hunters in the Dark) There are still three Halo Rings with unknown locations, any one of these could be the one Cortana took over.
She is connected to the Domain and has current access to human intelligence systems, so finding a forerunner facility isn't hard. Also, now that the Domain is active she can freely move between systems throughout the galaxy. She is not limited to one physical location at one time anymore.
It's pretty simple if you ask me. If you remember the story the consoles tell in halo 4, they show that the universe was once populated with both humans, forerunners, and other species the same. Only the humans were somewhat locked in battle with the forerunners against the flood. Humans had developed technology similar to what they have no in the halo universe. But the forerunners had to cleanse the universe of the flood in order to be able to save it. So they collected the DNA of ever species and put them in the same ships we see the master chief on at the end of halo 2. And once the halos were activated they wiped out everything including thenforerunners. Hence why in halo 4 the Knights actually had human remnants in them from a human civilization from before. So the ships dropped the DNA on their respective planets and civilizations evolved all over again. At the end when you see the new halo ring you here cortana humming the same thing the monitor from the first halo hums when floating around. And of course Laskys scene does parallel the first halo. From what I gather with everything cortana said and how she talks. And the fact that she said she would wake the chief in 10,000 years, can only mean cortana is now recreating exactly what happened the first time around only this time instead of the flood being the problem trying to consume the universe she believes humanity is. And it looks like she plans to cleanse the universe of all humanity and start fresh. I also have a small theory which I'm sure is extremely not true but does make you wander...... maybe the monitor from the first halo actually gained control of cortana, and that's why she's humming the same song when you see the halo ring at the end. But most likely its just paralleling the first halo.