I was wondering if all the resources (images, audios, videos) of an App are copied to memory when an iOS App launches, because if it is so then it wouldn't be necessary loading all the resources at startup to avoid any lags later.
No, they aren't, doing so would crash a lot of apps as the total set of resources will be too large to load all at the same time.
As a rule you should load only what you need and when you need it, or as close in advance to when you need it as you can.