[Jess Kitting]

Justice Alito temporarily grants Pennsylvania GOP request to enforce segregation of late ballots

Can one Justice do that singlehandedly?

[Oorang]

Yes, as this went before the Supreme Court late October.
https://www.supremecourt.gov/opinions/20pdf/20-542_i3dj.pdf

The Court basically said "come back to us after the election as no damage has been done at this time". Now the PA GOP has gone back and Alito responded as PA is in his district.