To get free courses topics wise, then you can try this website.
Udemy Quick Overview:
Ratings: 4.5 out of 5
Udemy Courses price range 9.99 to 199. They offer regular discounts that price is 10 to 12.
Udemy has free courses. Actually, it depends on the teachers. Teachers make it free for students. even the teacher can give free coupons. You can get premium courses for free this way.
The thing to keep in mind that default-free courses from Udemy don’t have quality.
Payment methods: you can pay through Paypal, Cards.