I’m not sure what your second question is asking but it sounds like you’re wondering if the definite articles and indefinite articles are interchangeable. They are not and they mean different things. Chris gave a good explanation of the difference between the definite and indefinite articles.
Furthermore, in French, nouns require an article before them which is often not translated in English and makes it a little confusing.
All nouns, except for cities as far as I understand, have a gender- masculine or feminine which you will learn as you go. And you have to use the correct form of the article for that gender. (Best way to learn this is to write down the article with the noun when you’re learning vocabulary.) The definite and indefinite articles have to agree in gender and number with the noun.
Definite articles are le (masculine), la (feminine) and les which is the plural for both masculine and feminine and for simplicity, are translated as “the.”
Je veux le livre (masculine) de la bibliothèque. = I want the book from the library. (a specific book)
Je veux la fleur (féminine). = I want the flower. (a specific flower)
If you want more than one specific book or flower then the noun would be plural and therefore, the definite article would also be plural. Les is the plural article for both masculine and feminine nouns.
Je veux les livres. = I want the books.
Je veux les fleurs. = I want the flowers.
The indefinite articles are un (masculine) and une (feminine) and are translated as “a or an” in English. The plural indefinite article is des and is generally equivalent to “some.”
Je veux un livre de la bibliothèque. = I want a book from the library. (Any book, just a book.)
Je veux une fleur. = I want a flower. (any flower, not specified)
If you want more than one unspecified book or flower, then the noun would be plural and the article would also be plural.
Je veux des livres de la bibliothèque. = I want some books from the library. (More than one unspecified books)
Je veux des fleurs. = I want some flowers. (unspecified)
I hope this helps.