Basically there are two parts to these animations. Or three really. There should be an animation for the first person model. One for the third person model, which likely can be the same one as the first depending on how you have the models set up. And lastly The weapon itself needs an animation of the mag being removed and replaced, typically in sync with the one from the first person model so the hand looks like it’s actually holding the magazine or ammo, etc so that when the animation is called for the weapon and the character model they are in sync.
For the weapon model specifically, you need a skeleton system on the model. Typically I use a small, custom skeleton for each weapon that consists of one bone per moving part. So for a typical pistol you’d have a root bone, a grip bone that has the frame of the pistol attached, a slide bone for the slide, a trigger bone for the trigger, a hammer bone for the hammer and lastly a magazine bone for the magazine. Then you place the weapon with the skeleton in the hand of the model you are animating and you can animate the magazine to perfectly line up and sync with the character reload animation. Same goes for firing and other animations.
Other than the actual animation the process to export a reload animation is the same for any other animation. Export as FBX and import to your model’s skeleton.