How do I access the 1040 that was filed in 2021?

How to Access prior year online returns
https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-access-my-prior-year-return/00/27010

If you can't get the side menus to open up to access the prior year..... You need to start entering some basic Personal Info in 2023 for the side menu to open up. Just continue a little ways into 2023. I had to go though about 12 screens.

If you used the Desktop CD/Download program then the only copy is on your computer and not saved or stored online. So you need to make and keep your own backups.

Or request a transcript from the IRS
https://www.irs.gov/individuals/get-transcript

Or get a copy of your return using form 4506
https://www.irs.gov/pub/irs-pdf/f4506.pdf