I would take a look at Platform Channels, which are a way to communicate Flutter code to platform native code (e.g. Java/Kotlin on Android, Objective C/Swift in iOS). From there, you can use your C++ library using the regular NDK mechanisms .
Can we use C in Flutter?
Flutter mobile can use the dart:ffi library to call native C APIs . FFI stands for foreign function interface. Other terms for similar functionality include native interface and language bindings.