is it common to use only indefinite article a or an for any word in the U.S?

Nope. ... That having been said, yes: "a or an ARE the indefinite articles" in English. However, they usually go (first and foremost) with nouns, then after that with adjectives and perhaps a couple of others. But indefinite articles are not used for ALL words, U.S. or any other English-speaking place... So -- am I not understanding something about your question?

It depends on our (individual) mastery of English grammar. You might hear "an apple," which is correct, or "a apple," which isn't and gives the Brits an opportunity to mock us.

The indefinite articles "a" and "an" are used before nouns which are fungible, without a specific item known or able to be identified. "A" precedes nouns (or the adjectives that go with the noun) that begin with a consonant sound. "An" goes with nouns that begin with a vowel sound.

Every day I eat a vegetable. (It's not states which vegetable the speaker eats.)

Every day I eat an onion. (But which onion? Nobody knows.)

That night I had an awful feeling. (It's "an" because of the adjective awful.)

That night I had a wonderful feeling. (No idea which feeling it was.)

He wanted an aspirin. (He didn't care which aspirin he got, though.)

She took a Motrin tablet. (They're all the same, right?)