Best methods of getting grocery coupons?

I know that there are plenty of toiletries and personal care items that have coupons, but I want to get into couponing for grocery hauls.

And please don't say it's not possible anymore, because I worked at a grocery store post pandemic when I was still in HS and I would constantly get "extreme couponers" who got most of their groceries for free. Some were even owed money from the store afterwards. I asked them how they did it but they weren't willing to share their secrets, fair enough I guess... But it's always been a dream of mine to be an extreme couponer since then.

What are the best sources for grocery coupons? Should I just get sunday papers? Email manufacturers? Is there a website that consolidates them?