Flutter is a reactive cross-platform mobile development framework created by Google, using the Dart language, which allows developers to write once and deploy on multiple platforms including Android, iOS, Windows, Mac, Linux, Google Fuchsia, and the web. It has several advantages such as hot reload, cross-platform development from one codebase, reduced debugging time, fast fluid UI, great app design, same app UI for older devices, perfect for MVPs, Flutter community and support, and open-source. However, it also has some disadvantages like learning a new language, libraries compared to native development, and app sizes. On the other hand, Kotlin Multiplatform is an additional feature of the Kotlin programming language that allows developers to use a single codebase to develop apps for both iOS and Android, with benefits such as modular integration, easy to learn, a single codebase for business logic, native UI experience, and native performance. However, it also has some drawbacks like being relatively new, community support and libraries limited, not a closed solution, and taking more development resources. Ultimately, the choice between Flutter and Kotlin Multiplatform depends on an app's needs, with teams that prioritize development speed over native performance favoring Flutter, and those that prioritize native performance and UI favoring Kotlin Multiplatform.