Tesla does not do traditional model years. Just because they were built in 2021 does not mean they will be 2021 model Year. I got my Model Y November 2020 and its a 2021 model year. Tesla switches their model year randomly and not at a fix date. There will be no different resale value based on the build date of Nov/Dec 2020 vs Jan 2021. Tesla makes incremental changes throughout a year so resale value can be impacted for various other reasons