I know very well a registered dietitian who deeply knows her stuff. She's explained quite a lot to me, and given me considerable knowledge (it helps that my field is chemistry, and while biochem is different than what I do it's not completely alien).

Unfortunately I can't say much about nutrition in one single post. Like so many things, it's a really complex and rich science and to really know something about it would take years of education on the subject. As you may imagine, everything comes with lots of exceptions and qualifiers. My recommendation if you really want to learn this stuff is to talk to a registered dietitian (any quack can call themself a neutritionist, but RD is a protected title), or read recent academic textbooks (NOT popular books) on nutrition.

About the subject matter in the main post, from my knowledge, meat and heavily processed foods tend to be more in the territory of things that are worse to eat a lot of. I can pretty certainly say it's true that many people would do better to eat considerably less of that stuff than they do.

Speaking from my area of expertise now, I'll say that it's pretty silly from a thermodynamics standpoint to eat meat. You can do fine as a vegetarian, and raising plants to eat is significantly more energy efficient than raising plants to feed animals to eat. Animal raising leads to a considerable amount of energy waste, as well as material pollution. I don't have the numbers on hand, but it's enough to be a significant factor. So what do I do? I don't eat meat very often.