In Eevee, everything is rendered in layers from back to front. If there’s a transparent material, the background is rendered and then the object is drawn on top of that. In what used to be the additive blend mode, that transparent layer was added to the background (think the add blending mode in photoshop or the add mode in the rgb mix node).
Example: background is RGB [0.1, 0.1, 0.1] (10% red, 10% green, 10% blue) and the transparent object in front is RGB [0.4, 0.4, 0.4] at 50% opacity. In Alpha Blend, 50% of the second color is averaged with the first, giving that pixel a result of
( [0.1, 0.1, 0.1] + 0.5( [0.4, 0.4, 0.4] ) ) /2 = [0.15, 0.15, 0.15]
With Add, you just add the two without averaging and that pixel would instead be
[0.1, 0.1, 0.1] + 0.5( [0.4, 0.4, 0.4] ) = [0.3, 0.3, 0.3]
Blend modes are pretty universal to most software, so you can find Add mode in just about every software that has to do with placing one image or video on top of another.
