Starting in v5.0.1 of 3D WebView for Windows and macOS, the native macOS library files (VuplexWebViewMac.bundle and VuplexWebViewMac_with_codecs.bundle) come zipped in a file named ZippedMacPlugins.zip, which 3D WebView's MacEditorScript.cs script automatically unzips when running in the Unity macOS editor. The reason for this change is that the Chromium Embedded Framework library used by 3D WebView internally contains necessary symbolic links, which Unity's unitypackage format and tooling don't support. Shipping the macOS plugin files inside of a zip file is a workaround to preserve the symbolic links while remaining compatible with Unity's unitypackage format and the Unity Asset Store. Without the workaround, Unity's asset export and import tooling creates copies of the symlinked files, which causes the macOS plugin files to become much larger than they should be (1 GB each instead of 345 MB) due to containing multiple copies of Chromium.
The MacEditorScript.cs script that unzips the ZippedMacPlugins.zip file runs only in the Unity macOS editor and doesn't run in the Windows editor. So, in order to build for macOS on Windows, it's necessary to first open the project on a Mac and save the unzipped files. The reason the plugins must be unzipped on a Mac is to preserve the symbolic links.