Dock Exposé is described as 'Window / task management app for MacOS. It provides previews on the dock using the familiar App Exposé interface (Control+Down Arrow). Trigger another application's (App Exposé) keyboard shortcuts (using Esc and Ctrl+Down-Arrow) to show/hide' and is an app. There are six alternatives to Dock Exposé for Mac. The best Dock Exposé alternative is DockDoor, which is both free and Open Source. Other great apps like Dock Exposé are HyperDock, ActiveDock 2, DockView for macOS Dock and DockMate.
Open source and featureful.