Thanks for the question, @dudoan1234.
@wolfdaddyspace is correct with regard to the distinction between Tweets and Scheduled Tweets (thanks!). From our Version 2 annoucement:
Scheduled Tweets do not become actual Tweets until the specified
scheduled_at time. This means that no other Twitter user can see them until that time; Scheduled Tweets will never appear in the Gnip Firehose.
Hope that answers your second question.
Next, let’s take a look at the relationship between Tweets and Promoted Tweets, your first question. Once Tweets are created, you’ll use the POST accounts/:account_id/promoted_tweets endpoint to associate the Tweet with a line item. This is done to promote Tweets in campaigns.
The POST accounts/:account_id/scheduled_promoted_tweets endpoint does the same, but for Scheduled Tweets. Hope this answers your third question.