When you update an app on iPhone?

Does it use memory (gigabits) when you update an app on iPhone? Like, is the app now "bigger"?

No. It basically deletes the app and reinstalls the new one. No memory harm done

Sure, the app can be bigger but typically space on phones is consumed far more by video/photos/music

Sometimes yes., when an app updates it can fix things that were wrong in it, like- slowness, glitching est. That may not take up GB maybe a few MB (megabites) but when they add new stuff like- new skins on a game or new maps est. It will take up MB but not GB unless you update alot of apps. Hope this helped.

Yes and no. If the update fixes an issue like a bug fix it might just add another line of code to the pre-existing app which would just take like a few more KB. If the app is a TRUE update then yes and no because the new installation might be cleaner and with shorter code, which would make it take up less. And then the other way around.
Hope that explained it well enough.

Just a by the way, Gigabits is different than GigaBYTES, you are probably talking about GB which is gigabytes, which a capital G and B which usually refers to memory. As GigaBITS, abbreviated Gb, lowercase B, refers to like router/internet speed.