I am a Lawful Permanent Resident (LPR) of the US. I wish to travel from Delhi to Toronto via air, and then cross by land border into the US. Will I clear US immigration when I land in Toronto? What documents will I need to cross into the US by road? Will I be fingerprinted again? Will it be at the Toronto airport or at the land border crossing?
On the itinerary you've outlined, you would enter Canada and go through its customs and immigration processes. You will then enter the United States through a land border crossing, of which there are more than 100 between the US and its northern neighbor. The one you select depends on your route and destination.
Lawful Permanent Residents of the U.S. must present a Permanent Resident Card ("Green Card", Form I-551), a Reentry Permit (if gone for more than 1 year), or a Returning Resident Visa (if gone for 2 years or more) to reenter the United States.
The information provided by other users is correct, however, I will try to provide better clarity by answering this question more generally:
Canada and the US are separate countries. If you fly from a third country to either Canada or the US, you must meet the entry/transit requirements of the country you are flying to. If you travel between Canada and the US, you must also complete border formalities for the country that you are entering.
Arriving and Admissibility to Canada
Since the outlined itinerary has a stop in Canada, you must satisfy Canada's admissibility requirements. In the case of an Indian National with a US permanent residence card, you must obtain an eTA. However, if you are ineligible for an eTA (for example, you are an Indian National without a US permanent residence card), then you would require a Canadian Visitor Visa. Note that a Canadian Transit Visa or the transit without visa program can not be used for the above itinerary as a traveler must both arrive and depart by air in order to use those programs.
US Border Control Location
In most cases a person travelling to the US by air will complete US border formalities shortly after landing. However, the US Customs and Border Control maintains some preclearance facilities which allow passengers to clear US border formalities before physically entering the US. Many Canadian airports are equipped with these facilities allowing US bound passengers to clear formalities before flying. However these facilities can only be accessed by travelers prior to boarding a US bound flight. Travelers who are travelling to the US using any means other than a US-bound flight can not use or interact with these facilities. If you are travelling to the US by land, you will complete US border formalities shortly after physically crossing the border. In general, US land border formalities are similar to the formalities that one would experience when arriving by air however, certain documentation requirements are lessened for land boarder crossings. Nonetheless, if you would normally be fingerprinted when arriving by air, one should expect that you could be fingerprinted at a land crossing.