Install
Atomics installs from the browser you are reading this in. There is no store, no download, no review queue and no account — the page you already have becomes the app, and opens in its own window with no address bar around it.
Offline
Installing puts the app on the home screen. It does not, by itself, mean every page and every library is already on the device — some of it arrives the first time you open the part that needs it. This downloads the lot up front, and then lets you switch the network off inside the app to check.
Airplane mode holds every request the app makes, whatever the connection is doing — the same thing a tunnel does, on purpose and from a chair. Anything already downloaded keeps working; anything not, does not, which is the point. It stays on until you turn it off, in this window and every other one open on this app.
iPhone and iPad
Safari is the one that does this on iOS. If you are reading this in another browser on an iPhone, open atomics.app in Safari first.
Android
Chrome usually offers this by itself. If it has, the button at the top of this page is the whole job.
Mac, Windows and Linux
Which is the half the old version of this page left out. A desktop install is the same application in a window of its own, with the same spaces in it.
Questions
It runs in the tab you are in. Installing only changes the window it opens in and what happens when the connection goes.