It’s depends on which developers you are targeting and what you are trying to develop.
The majority of developers will probably use native solutions, Objective-C /Swift for iOS and Java/Kotlin for Android and the is no way of creating a single library for both unless you develop C/C++ code but this would probably be problematic at best.
There are multi-platform frameworks such as React but you will be targeting a specific set of programmers for this.
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solved what is the programming language for SDK development for Mobile app