I know this because I originally planned to buy one of these 'new' with miles model Ys until I heard about the improvements coming to the 2021. The new models are typically loaners/demo vehicles. Used are Cars that were new but had too many miles (some states have max miles allowed on a car to be sold as new) OR they were actually bought by someone and returned during the 7 day window, I'm sure there are other possibilities as well. Maybe someone bought a car and then immediately traded it back in.Ordered 11/1 MYP, MSM, FSD, Hitch and have no VIN, but estimate 12/13 to 12/31.
What has me so curious is that you can poke around the Tesla website under new and used inventory and see Model Y's with as low as 100 miles and many under 1,000 miles with various option levels available for immediate order. Even stranger is that under new it has Y's with hundreds of miles and used with the same mileage, so how is New or Used differentiated. Check this out New & Used Electric Cars | Tesla
If you go an test drive a car, basically any of the test drive demo cars can be purchased. They don't advertise them all under the new section but if you particularly like one of the models you drive you can as your SA about purchasing it. That's partially why I go to test drive once every 2 weeks or so haha.