How can green card holders apply for re-entry permits outside the U.S.？

If you out for more than one year the green card is deemed abandoned and you cannot file a reentry permit until you are physically on U.S. soil. You making a mistake if you do not return before one year.

You may need to contact the U.S. embassy or consulate for a returning resident visa. COVID-19 has created a lot of issues like this.

No, but you can file for a transportation letter at the consulate.

It is not possible to apply for a re-entry permit from abroad. There are various things to consider about a prolonged absence and you should discuss these with your lawyer. Given COVID, many are choosing to stay out of the U.S. for longer than initially planned.

You cannot do that. You must be inside the U.S. when USCIS receives the re-entry permit. What you can do is to go to the nearest US embassy or consulate and ask them to give you a Returning Residents Visa. That will save your green card. You may be able to apply for that online at the consulate.

You cannot apply for a re-entry permit outside of the United States. You can apply for an I-131A transportation letter if your absence is less than one year or an SB-1 returning resident visa if your absence is over one year. You should be prepared to document your ties to the United States (driver''s license, property ownership, employment, medical/school records, credit cards, bank statements, and tax returns) and you should also document your reasons for not being able to return to the United States (canceled air reservations, a letter from your doctor advising against travel, etc.).

You cannot get a reentry permit outside of the U.S.