There are literally many browsers available on iOS. Some of my favorites are Dolphin and Atomic. However, you are right in one sense; Safari has a "favored nation" status in iOS such that some default browser situations will always use Safari.
Also, at one time, Safari was allowed to use some features of the iOS that other browsers were not allowed to. This gave them some performance advantages back then. I think that is no longer the case after a large outcry.
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