What specific internal data type is used to store text in string variables?

In iOS, the class NSString (in Objective-C) or the structure String (in Swift) is used to store text in string variables. Internally, text is stored as a sequence of Unicode characters, usually in UTF-16 format. In Swift, String is optimized for working with Unicode and uses a complex internal structure that may include buffers with UTF-8 or UTF-16 encoding, depending on the situation, for efficient storage and processing of text.