I am citizen of India (Indian passport holder) with a valid F-1 visa and work permit (EAD card) with me. I have an Air Canada flight from Delhi to LA via Toronto so I have a 5 hr transit at the airport. I am not planning to go out of the airport. Do I still need a transit visa or eTA to board my flight to LA? If yes, which one do I need? I have already checked this link and doesn't specify what type of visa do I need (eTA or transit). Also, it doesn't take into account that I am a valid US Visa holder.
Yes. You need to apply for a Canadian Transit Visa. From the Immigration and Citizenship Canada's website
You need a transit visa if you are from a visa-required country and:
your international flight stops at a Canadian airport on its way to another country
you will be connecting between two international flights at a Canadian airport
Some people can transit through Canada without a visa if they are travelling to or from the United States. Find out if you can transit without a visa.
Since India is not a Visa-Exempt country and Indian citizens are ineligible for Transit Without a Visa you are required to obtain a transit visa.
Other users have pointed out that there exists a sterile transit corridor at Toronto Pearson and that you do not need to present yourself to a Canadian CBSA agent during your transit, however both your airline and the US CBP agent will likely enforce the transit visa requirement.
The airport has a handy connecting guide, you should input your airlines there to see your requirements. It appears that as long as you arrive and leave through the same terminal, there is no requirement to present yourself to the CBSA (Canadian Border Security Agency), only US CBP.
I have checked International arrival on Air Canada to US departure on Air Canada, and it appears that that is OK.