If you want them healthy, their primary feed should be hay - Timothy is usually best, if you can get it. Orchard is next best, alfalfa should be a treat. Do take care what you feed them, as far as greens. Raisins, grapes, avocado, garlic and onion are highly toxic to them, and there are other things, as well. Most greens are good, but go very easy on fruits, carrots, and sweet potatoes, because the sugars are no better for their health than yours. Watch their poop, to determine whether you're giving them too much fresh produce. It should be all pellets, and just barely moist. If it's not shaped in small, firm pellets, they're probably getting too much produce and not enough hay. Pellets are more expensive, and don't provide enough roughage for their very delicate digestive systems.
They'll also produce tiny, oily cecotropes, that look like tiny, clumped up poop. But, they need to be able to eat those. This should help:
https://www.rabbitcaretips.com/normal-and-abnormal-rabbit-poop-types/