Hi Patton9 and welcome!
You can capture a base by destroying the nearby town and either driving or flying 10 troops to the maproom in said town. You can ask for more details using the country radio inside the game.
AFA Clans, they're called squads here. Some existing squads are actively recruiting, some are more restricted to their current personnel. In any case you'd need an invitation to be able to join but asking is free.
The only "upgrade" option is to earn Perk Points that allow you to fly certain planes until you crash them or run out of perk points. They're not a shortcut to championship, though. A Me 262 jet can't be flown like a Brewster... The weapon options for each plane is what was used in them in WW2. For gameplay purposes the guns don't get jammed like in real life, but the velocity, bullet drop etc. are modeled quite accurately.