How exactly are the bottom bar in iOS and Android different?
Please give specific examples ..also exceptions and why the exceptions exist.
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Google's guidelines for a bottom navigation bar are relatively new. It is believed they were against the use at first, because people could easily hit the wrong button with the touch sensitive hardware buttons at the bottom of most Android devices.
They never gave information about this, but my guess is they noticed just having the hamburger menu was hurting the discoverability of the app and the tabbar at the top wasn't sufficient for a lot of situations, plus it was hard to reach.
So much for the background information. You're asking what the exact difference is. I suggest you take a look at the guidelines for both Google Material Design and Apple.
You'll notice Google suggests icons in combination with text labels and certain animations on active pages.
Apple suggests either icons for multiple links or text labels for three or less links (text is less likely misunderstood / more clear).
And that would be the biggest difference. There's more, but you can read that for yourself.
But be aware, these are just guidelines. When designing an app, you stand before the choice of designing the app platform independent so the style is uniform across both Android and iOS for brand recognition and your own full custom user experience. Or you style it according to the platforms' guidelines, creating an app that will be more intuitive for the user since they're familiar with the design patterns native to their phone.
Airbnb has the same components and design across Android and iOS.