Do I need to cancel my ED visa to apply for a Non-B visa and how can I apply for a Covid-19 visa?

You get documentation from your school stating you stop studying on xxx date, you go to the immigration office a few weeks before that date, cancel the extension you're on and then (if it's before Nov 26th) immediately apply for a 60 day covid extension.

Now whether you can get an in country Non-B visa issued at the immigration office you use is between you, the company you will work for and the immigration office you deal with.

You need to be talking to your future employer to see if they can get a 90 day Non-B visa issued from a covid extension.