I remember atriox was stuck on the ark, so how he reached the infinity???
His men built a ship or something.
Or they called reinforcements.
In Shadows of Reach, there was a portal he used to get from the Ark to Reach. That’s why the Banished were there. To find and activate that portal.
The Banished found something on the Ark which made him realise Zeta Halo had something even more important than what he had sought on the Ark, hence his efforts to return to the Milky Way
On the Dreadnought that The Prophet of Truth took to the Ark in H3 were fragments of a Forerunner slipspace crystal that could bend the very fabrics of space and time. Well Atriox killed the remaining Covenant loyalists in the Dreadnought and took those crystal fragments. He was then able to use the fragments (while on a Lich) to travel to Reach and met with Escaharum.
That is how Atriox was in our Galaxy and how he was then able to get to Zeta Halo in time for the beginning of Halo Infinite. Hope this helps 