A campaign is one message sent to one segment, by email or by SMS.
Five states, and one of them is a one-way door
Draft → scheduled → sending → sent. If a send fails, the state becomes failed.
While it is a draft everything can be changed. Once sending has begun there is no going back: a sent campaign cannot be edited or sent again. To send the same message again, make a new campaign.
The reason is straightforward: some recipients already have the message, and changing the text does not change what was actually sent — it only makes the record untrustworthy.
Recipients are decided at send time
A segment is not a fixed list of people. It is recalculated at the moment of sending.
So the number you saw while building the campaign is a forecast, not a contract. Schedule a campaign for tomorrow and it reaches whoever matches the conditions tomorrow.
Sending happens in the background
Messages are queued and a background worker sends them. That is why "sending" can persist for a while and why the statistics climb gradually.
Scheduled campaigns work the same way: the time you set means "from this point onwards", not that exact second.
The campaign body is built in the editor
The email is composed with the same page-building tool, but what it produces is not a web page: the email is built from a simpler structure so it survives Outlook and Gmail.
Which means layouts that are possible on a web page are not always possible in an email. That is a limit of email, not of the editor.