The WebView is an OS provided library used by apps for rendering web-based content internally. It's most commonly used for the intended purpose of rendering small bits of web content locally. It has JavaScript disabled by default. Email clients rendering HTML emails is a very common use case. It can also be used to essentially run a web app inside of a native app instead of making an actual native app. It can be used to make a hybrid between those due to the ability to define JavaScript interfaces within the web environment from the app's native code. It can be given access to the app's files or content handlers for local usage. It's just a way to use web standards to write parts of a native app or to render web content such as HTML emails, websites, etc. within an app.
Our PDF Viewer is a good example of an app heavily using the WebView with the original intended purpose of rendering content with web-based technology within a native app. See https://github.com/GrapheneOS/PdfViewer/blob/main/README.md for an explanation.
The WebView is not the Chrome Custom Tab interface where an app can open a tab in the user's chosen browser over top of themselves with minor customization of the theme and the option to move the tab to the browser's activity. Many people have the misconception of that being the WebView.
Does WebView always use Vanadium, can the user modify this?
Vanadium WebView is the only one permitted by GrapheneOS, similarly to how the Chrome WebView is the only one permitted by the stock Pixel OS and Google Mobile Services operating systems in general. It would be a huge violation of the app security model to permit arbitrary WebView providers since it's given access to data of other apps through them using it as part of their app's implementation. It's an OS component. There's a system for installing other OS approved WebView implementations but that's meant for use cases such as using a WebView provided by the Chrome Beta channel rather than a third party one.
no masking of common fingerprint techniques?
That's not accurate and having multiple WebView implementations would make fingerprinting far worse for people who switch away from the standard one. Not really an issue with normal uses of the WebView though.