This page of the Halo Infinite guide contains a walkthrough for story mission 4 - Recovery. The following solution explains, among others, how to unlock the world map (Halo Zeta ring), how to take over FOB bases and how to reach the Tower, which is the source of the signal .
Establish a Forward Operating Base
This task starts immediately after completing the previous mission (Outpost Tremonius), i.e. after securing the landing zone and using the console. Once the console is activated, a flying machine piloted by Echo 216 will appear.
Get on board to complete Outpost Tremonius - you'll be taken to a different area of Zeta Halo. New, yellow markers will appear on the map, indicating new objectives. Moreover, you can now freely explore the world map discovering secrets, or engage in optional activities. . You may take a special interest in taking over Ransom Keep, as completing this activity yields 100 Valor points. All side activities are described in detail in a separate section of the walkthrough.
Two of the three yellow markers symbolize FOB bases. These are small outposts guarded by relatively small groups of opponents. It is worth attacking them by surprise (preferably from a suitable vantage point).
After you have eliminated all enemies from a given FOB, you can interact with its console to capture the base. From now on, you can summon vehicles (initially only Mongoose is available) and equip weapons unlocked with Valor points. FOB bases also serve as fast travel markers.
Investigate the UNSC signal
The third yellow marker symbolizes The Tower and this is the location to which you must eventually go to push the main plot forward. You can get there alone or with allies (find a vehicle to drive them) which will facilitate further battles.
When you approach the tower, the next story mission will automatically begin.