How Stello works
Stello creates encrypted messages and then sends links to them with the password included, so recipients simply need to click the link to decrypt and view the message. Recipients do not need to install any software as the messages are displayed as standard webpages.
The links to Stello messages then need to be sent to recipients. Stello can do this automatically for you for email contacts, but you can also manually send the links to contacts using any other messaging platform.
Anyone who wants the nitty gritty of how Stello works can simply come take a look as Stello's source code is publicly available.