Yes, lowering opacity is probably what you want.

It may also work to increase reflectivity? (I've not tried this.) They may use other terms, like "alpha" and "metalness", etc., depending on your software. This depends on your game engine. (I believe I know how to do it in Unity.)

You can probably handle all buildings in the scene (if they use the same glass) by editing the material &/or shader.

So there may be more than one way in Unreal, Blender, and even Unity. (I'm just a hobbyist at this point.)

make them less transparant by lower the value of your opacity map