By the power of truth, I, while living, have conquered the universe.
The Covenant begin glassing the area of Voi to reveal the Portal Artefact that will take them to the Ark.
Virgil likely provides the UNSC with information regarding what the Covenant are digging for (though obviously doesn't know an awful lot about it, as nobody knows what it is in Halo 3). As for Buck's squad, they either:
1) Went through the Portal to the Ark and fought in the ground battle.
2) Stayed at Earth to protect it from the Flood when the Gravemind arrived to go through the Portal.