Why do people say "How to use an iron" meaning any iron? Doesn't it mean that there is one iron and you ask how to use it and that because we can't say "I love a cat" because it means one cat? Shouldn't it be "How to use the iron" meaning not a particular iron but any iron in the world or rephrased with the plural form. And why in the "A X is a Y" or "A X is…" (for example: A dog is an animal) sentences the indefinite article is used as "any" but not as one of the class of X. I can't say "Cigarettes are bad for a body" (with meaning every/any body) but I can do that in the first example with an iron generalizing the word.
Normally you use "a" or "an" to mean one when you are not referring to a specific one, and you use "the" when you are referring to a specific one. So "I will show you how to use an iron" is general - any iron would do. "I will show you how to use the iron" would usually refer to a specific iron.
There are exceptions. In "I will teach you to play the piano" we use "the" even though it applies to any piano.
Plural forms can be confusing. "I can play castanets", because I use two together. If you say "I can use irons" you invite a reader to ask why you used the plural. Do you use more than one? Are some different from others, needing different skills?
Some of your other examples are correct but might not mean what you want. Context is important and the same construction can have different meanings. "I love a cat" means there is a cat somewhere that you love. "I love the cat" means you love that specific one. If you used "own" instead of "love" the constructions work the same way. But while "I love cats" means you love all of them, "I own cats" only means you own more than one.
"Cigarettes are bad for a body" is not used because using the indefinite article suggests there are more than one. Using "the body" refers to the only one you have. "Anybody" and "any body" are not the same.
You have pointed out some interesting variations in how the articles are used. I think the best strategy is to hear and read as many as possible - the rules are not always clear.