This. Airlines have to pay to keep their options open. I doubt they have a specific place in the queue but they may say something like 'we'll potentially take six in 2026 and six in 2027.'

Airlines are always going out of business, cancelling or deferring orders etc so when you read about the A320 or B737 being sold out until the end of the decade it doesn't tell you the whole story.
In the past few days Airbus have just had their Go First order cancelled and a few frames are already built so there are some A320s for someone else to pick up almost immediately, the same with Boeing's assembled MAX's for various airlines that have been taken up by United, Qatar etc.
On the A350, Air India have acquired some that are already assembled for Aeroflot. Although the order was announced months ago it was only confirmed at the Paris Air show last week after the airline secured funding. They''ll be receiving a few whenever the interiors are redone. Having said that, BA/IAG generally don't buy aircraft that were made for other airlines but never say never.