Below are all the collectibles found during the Nexus mission and The Command Spire mission in Halo Infinite.
Nexus Mission
UNSC Audio Log #1
Obtained automatically as part of the story when you receive the objective to power the 3 crucibles.
UNSC Audio Log #2
This log is found when you’re looking for the 3 power crucibles to power the grav lift. Head through the middle door (when facing it, the red door will be behind you) to find the log on the left side of the room next to a UNSC duffle bag.
The Command Spire
Skull – Mythic
As you proceed through the mission, you’ll come across pillars that emerge vertically from a yellow energy barrier. From this point, keep an eye out for the pillars that are moving horizontally. Head past 2 walls of the yellow barriers to reach the end of the room where the pillars will turn right and be sent upward. In the room where they turn right, there will be an open square section in the ceiling. Grapple up there to find a door that leads to the room containing the skull.