There are two ways to get dev tools for Anki.
AnkiWebView Inspector (easiest method)
Install this add-on: AnkiWebView Inspector - AnkiWeb
and then right-click on any webview (in your example, the preview window) to bring up the context menu and hit “Inspect”.
Note: In the current version of the add-on, this will open a dev tools pane on the main window of Anki. In the next version, it will open directly in the window you clicked “Inspect” on.
Remote debugging
Start Anki with the environment variable
QTWEBENGINE_REMOTE_DEBUGGING=8080
and then open an external browser (e.g. Chrome) and navigate to http://localhost:8080/
. There, you’ll find all open webviews.