Actual seat inventory is commercially sensitive and usually internal to the airline.

Even if you could get this information, you may not find it helpful in reality. You would need other information too (for example, what is the average rate of cancellations / no-shows for this particular flight, are there any special events taking place that might change the no-show/cancellation rate, etc). Especially in premium cabins, people change their flights a lot - I have a trip on CX later this week where I must have changed the ticket 5 times in the last 3 weeks.

Airlines like CX have whole teams of people who take the actual inventory available at a given time, and a lot of other factors, and then produce the numbers you see on EF.