Resident Alien but outside of US most of 2020 because of Covid-19

Since you don't pass the Substantial Presence Test (due to being in the US less than 31 days in the year) or the Green Card Test, it seems that you would by default be a nonresident alien for all of 2020 (unless you are married to a US citizen or resident alien and choose to be treated as a resident).

I think this also means that the salary for your work from the part of the year you were outside the US should not be subject to US tax, since it's foreign-source income (income for work performed outside the US) of a nonresident alien. Your W-2 should only list income from the part of the year you were working in the US, and if it includes more, you should get your employer to correct it.