[Martin Malmqvist]"I want to learn how the animation preset "Linear Wipe" works since it has an alpha channel built in to it. How can I create this for my own animation presets?"
the preset for linear wipe is just the linear wipe effect with 2 preset keyframes in the transition property. it's the effect itself that creates the transparency, not anything in the preset.
if you wanted to create your own preset with the linear wipe effect, add the effect to a layer, set the angle and feather properties as desired, then add keyframes for the transition property to create the wipe.
in the timeline select the properties that have been modified for the linear wipe effect (shortcut: type 'uu', that's 2 u's quickly, to reveal modified properties) and choose animation>save animation preset. give it a name and file path, and then you can apply those exact settings on any layer in any project from the effects & presets menu.