I can't see YouTube videos due to restrictions on the network where I work, but from your description it sounds like what's referred to as light leaks or film flash. There are several ways to do it. Artbeats' film clutter collections have stock footage of light leaks you can lay over your footage with various blending mode. You could create some gradient images yourself to copy the look or you could follow some of these tutorials:
"I find the Light parameter in Shatter pretty awesome for such effects. Unlike Levels (keyframed and applied to an Adjustment Layer), which is another popular method, Shatter's Light property has color AND direction. The direction property makes the effect so much more effective.
Of course you will want to switch off Shatter's primary properties so that shattering doesn't occur." - Roland R. Kahlenberg
There are a few 3rd party effects that will do these kinds of effects on a single layer. To build the flash from scratch you will need to create a gradient that you can animate to use as a track matte for a solid that you put over your footage using a blend mode like add. You'd animate the opacity of the solid. You'd also need to duplicate the animated gradient layer and put it on top of your footage as a luma track matte for the footage. A white solid would be placed at the bottom of the timeline or the composition background could be changed to white to complete the effect. Your timeline would look something like this:
In this example the hotspots.png is the footage layer.