You won’t be able to copy a 2019 from another computer to a new computer, and get it to work, because you won’t be able to add the license. That’s because your 2019 was deactivated when you started using the 2020 version of the license. The 2020 license was deactivated when you started using the 2021 license.
You should be able to copy SketchUp 2021 from one computer to another computer, and when you run SketchUp on the new computer you would add the 2021 license.