Quality assurance

Widgets are normal apps which run together with other widgets or one app

More or less. They are just bits of JS code (xyz.wid.js) that run and then insert themselves into the WIDGETS array so they can be layed out and rendered at the right time.

A Widget must be running (normally meaning: shown on the display) to be able to do something (Example: no GB widget, no notifications)

Yes, absolutely. If you want it running all the time it can be in xyz.boot.js but you have to be careful. An app may not run widgets because it doesn't want anything to interrupt it :)

Some are loaded automatically during boot (here I do not know what when happens exactly)

xyz.boot.js are loaded automatically during boot. However all widgets are loaded in response to a call to Bangle.loadWidgets(), which any app that wants widgets needs to call.