Straight answers
The web app is what launches, and it is the whole app. It runs in a current Chrome, Edge, Firefox, or Safari on a desktop-class machine. A Windows desktop application is ready but not yet released, and Mac and Linux are not far behind. Your license covers the desktop application when it ships, at no extra cost.
Federal is the complete surface. State income tax is early: a fifty-state rate dataset is in, a handful of states compute a modeled dollar today, and the rest are coming. Where a state is modeled rather than read off a filed return, the app shows the modeled figure next to the filed one.
The web vault lives in your browser's storage, which the browser itself can evict if it runs short of space. That is the honest cost of a product with no server holding your data. Keep the encrypted backup file somewhere you already back up, and treat the hosted backup as the belt to that pair of braces once it is switched on.
The backup page explains how the encryption protection works.
Trust, but verify.