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All of their lectures are uploaded on YouTube for free if you only want to learn by them
And if you want a certificate too as a part of course curriculum then register yourself on their portal -> choose the course you want(nptel swayam portal) -> start taking lectures (you have to open them atleast to mark attendance) -> register for its exam by paying for it -> score above 40% to get the certificate
PS: NPTEL exam marks = 75% of final exam marks + 25% of assignment marks