3D WebView for Windows and macOS embeds Chromium, the browser engine that powers Google Chrome. When an application creates its first webview, 3D WebView starts Chromium, which show up in Task Manager as multiple processes named "Vuplex WebView.vuplex":
Chromium uses multiple types of processes (e.g. browser process, render process, GPU process), so there are multiple "Vuplex WebView" processes for each webview in the application. After a webview is destroyed, its processes are removed, but one or more "Vuplex WebView" processes still remain running because 3D WebView keeps Chromium running until the application terminates. If you want to manually terminate Chromium while the app is still running, you can do that with StandaloneWebView.TerminateBrowserProcess().