How is the issuer working with the app?

How can you register as an issuer? Is the same app for issuers?
How is the communication with the recipient? I mean, How the issuer finds all the recipients (I didn’t add my e-mail in the registration proccess)?
What algorithm does Blockcert use to convert the seed passphrase into the keys?

The single use code, what is it? What is it for? How does it look like?

Does anybody have any ideas?