Hi Delta6NKVD,
It would be the .def files your looking for.
However if you look in any of the variations of the Su-25.def files you'll notice the "{## Weaponry" is commented out.
I assume this is because the weapons for planes has not yet been fully developed.

You could try removing the "##" from the line and replace the weapon types with weapons from other vehicles just to see what happens.

How do you make planes even work!? I cant even get in planes.

But yeah, you change weapons in the .def file. You want the section that looks like this:

{extender "inventory"
{box
{item "ammo bullet105 ap" 30}
{item "ammo bullet105 fg" 10}
{item "ammo bullet105 heat" 5}
{item "ammo 7x51" 3000}
{item "grenade decoy" 6}
}
}
{Weaponry
("restore_ik_time")
{place "gun"
{RestoreIKAfterAim}
{weapon "105mm_m68a2e1" filling "ammo bullet105 ap" 1}
{charger "gunner"}
("abm_high")
}
{place "mgun2" ;coax
{weapon "m240_v1" filled}
{charger "gunner"}
{foresight "foresight5"}
("abm_mgun")
{basic}
}
}

Thanks guys.