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Creating a C#/Typescript GUI for a C++ library
(programming.dev)
Icon base by Delapouite under CC BY 3.0 with modifications to add a gradient
Depending on where you want your app to run, QT framework/creator has pretty powerful C++ GUI capabilities. The biggest benefit, I thought, is a lot of open source projects use it (e.g. Bitcoin and forks) meaning there are a lot of real examples out there rather than only docs.
The other option, which is more aligned with separate frontend/backend and WebApps, is simply have a basic server of some type wrapping the dll functionality you want to use as endpoints and communicate over ReST API.