For Pyinstaller you need to include either WebBrowserInterop.x86.dll or WebBrowserInterop.x64.dll depending on whether you build against 32-bit or 64-bit Python. The DLLs bundled with pywebview and are located in the
site-packages/webview/lib directory. There is currently an effort to provide a hook for pyinstaller that would bundle this DLL automatically, but for now you have to resort to this step.
TODO: add Linux freezing guide