Zeal is described as 'Free and open-source offline documentation browser for developers. You download docsets for the languages, frameworks, and libraries you use, and Zeal lets you search across all of them at once and jump straight to the symbol, class, or function you need' and is a very popular documentation browser in the development category. There are more than 10 alternatives to Zeal for a variety of platforms, including Linux, Mac, Web-based, Windows and Flathub apps. The best Zeal alternative is DevDocs, which is both free and Open Source. Other great apps like Zeal are Code Wiki, Dash for macOS, Velocity and Zest.
They are basically twins, since both find documentation by pressing on a keyboard shortcut.