Wing and fuselage gun loading is highly dependent upon the particular installation. Typically, filled cans were installed and then leader linked belts were attached and fed thru the feed housings to the guns. Some fixed installations, the loose rounds were fed into the magazines by hand or machine. Expended brass was typically shed overboard via discharge chutes. More modern installations do not discard the brass, but have a capture mechanism and it's downloaded during a rearm of the weapons.
Until the advent of the MAU-12 type bomb release, weapons were simply allowed to free fall from the suspension system. Due to airframe configurations, some required the weapon to be hung from a trapeze. When released, that would force the weapon away from the fuselage in an arc, to clear the prop. These would be seen on aircraft used mainly for dive bombing, with center hung ordinance.
The MAU-12 style release units use either explosives or high pressure gas storage to drive a separation piston or pistons against the weapon to force the weapon away from the release unit and into cleaner air, at a nose down angle to enhance separation. These release units came about as release airspeeds routinely passed through about 400 kts.