You end of year inventory for 2020 is your beginning of year inventory for 2021.
Did you have inventory on hand at the end of 2020 (or did you sell all of your inventory)?
Thanks, @Anonymous_.
Yes, got that---I understand the basic premise of EOY last year being Beginning of Year this year.
The problem is something else.
Whatever TurboTax directions I followed last year resulted in the 2020 EOY inventory value field being automatically populated in Beginning of 2021 as... blank. Not even a 0. Maybe I didn't have an inventory value last year? This business has been more of a side income thing, gradually evolving into a more established source of income, and perhaps last year I chose a more simplified method.
As far as I can tell, I should perhaps choose simplified options only this year and see if it keeps me out of questions that are difficult to answer.
If you are a small business taxpayer (most likely), you are not required to keep an inventory.
See https://www.irs.gov/publications/p334#en_US_2021_publink10005555
The TurboTax expert who spoke with me on the phone helped me through it. She gave a different answer from yours regarding the necessity of doing so this year, and how to present the information properly. Thank you for the link but it was no copacetic with my situation.