how to see all media in one place in an iphone
Apple iOS runs each app in a sandbox, so in general, no app can see data from any other app. Enhanced security and privacy on iOS over android does come at a cost of data interchange. This design decision also enhances performance and saves battery life since every app isn’t trolling all the others for updates or plundering data it can then upload in efforts to track everything you do and store on your device. 

That being said, there are several common file storage API on iOS. If every app you use did store the video/image/data in a common place, there’s no technical reason why you couldn’t have the same unified view. 

In practice, you’ll need to manually curate or save off media you want on a case by case basis since most “free” apps like Instagram, Snapchat, Facebook have business models that make sharing content against their best interest. Since consumers are the product instead of the customer of those apps, I don’t foresee huge or quick adoption of common storage on iOS anytime soon - even with iOS 11 making it far easier to use the “generic searchable file system” idiom you mention that common on computers and android os.