Can I upload data into turbotax online from a return prepared using turbotax offline (after the extension deadline passed)?

YES!   You can transfer from the prior year on your computer into the Online version. You will need to have the .tax2020 file on your computer. If you used the Desktop program last year here’s How to transfer into the Online version, it may say 2019 on it but it means 2019 like the .tax2019 file........
https://ttlc.intuit.com/community/tax-topics/help/how-do-i-switch-from-turbotax-cd-download-to-turbotax-online/00/859756

If you already started an online return you will have to clear it and start over to transfer. How to clear and start over
https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-clear-and-start-over-in-turbotax-online/00/26444

If you used the Desktop program before you may notice several differences and not like the online version. It has less features and no Forms Mode. You can only do 1 return. Your return is only saved online so be sure to save both the .tax2021 data file and the pdf file to your computer when you are done. In case you need to amend in the future or if you need a copy to get a loan or something. You might not be able to access your online return when you need a copy.

I just completed my 2020 return using the offline version of TurboTax. Can I upload the data from that return into TurboTax online,

Unless things have changed that I'm not aware of, understand that you can not upload the .tax2020 file from the desktop version of TTX 2020 to your online account and then import from it with the online version of 2021. When you start the online version of 2021 it will give you the option to import the data from the .tax2020 file on your computer.