First, understand that these are not staged missions with AI. We do not yet have that capability for AvA and there is no guarantee that there will be anyone in an opposing mission.
Some missions like combat air patrol or defensive missions are useless there are players flying the opposing mission. Others, like target recon and attack missions you can still accomplish even if there is no opposition.
The briefing is in the title of the mission. Example: 1A Brief @V6 Oclub = mission 1 allies briefing is at v6 oclub
You simply go to base V6 Oclub, and there is your briefing. You can do this prior to joining a mission or after.
Missions do not necessarily launch from the same base where you go to read the briefing.
As long as you do not take up another plane, you will launch in the mission regardless of which base tower or Oclub you are at.