The answers I’ve seen are quite clear, but in a short way to explain it: it depends on the emphasis of the noun. “a” means something/someone random. Example: A girl is here (ok, a random girl). “the” is used for something/someone concrete. Example: The girl is here (could be the girl you like or whatever reason that makes her unique).

Thank you all! I appreciate your answers.

No, it's not a sporadic reference, and to be honest, terms like this aren't worth explaining. Believe me, the vast majority of technical terms connected with language are actually useless. Nobody needs to know what sporadic references are. It makes me weep to think of students the world over, wasting their time worrying about pointless terminology like this in the vain hope that it will help them understand English. But back to your question... We use 'the' in these sentences for the same reason we use 'the' in any other sentence, and it's something very simple. We use 'the' whenever it is clear to the speaker and the listener which particular beach, bus or doctor we are talking about. 'I went to see the doctor'. This implies that I went to see my usual doctor. Maybe it's the doctor I always see, or the only doctor in my neighbourhood. 'I went to the beach' might mean the beach connected to the town that we are talking about, or the beach which is nearest to your home. 'I read the newspaper' suggests that it's the newspaper you usually read. 'I take the bus to work' refers to the particular bus service which runs from your home to your place of work. Other phrases like this are 'I go to the gym after work' or 'I go to the pub every night'. Here we are talking about the gym that you are a member of, or the pub which you usually go to. If you were to say 'I go to a pub every night' it would suggest that you go to different pubs. If you say 'I go to the pub every night', we'd understand that it was your local pub.

I believe sporadic reference is a type of generic reference. The reason we use "the" instead of "a" doctor is because normally when you see a doctor, you see a specific doctor. It doesn't matter which doctor. However, you could also tell someone which doctor exactly that you saw. This is both generic reference and sporadic reference. The same applies to beach, newspaper, and bus. In all of these cases, you could say "a" instead of "the". Not all generic reference is sporadic though. "The beach is a good place to relax". In this case there is no specific beach. You cannot tell someone which beach specifically, because you mean all of them, but a person cannot visit every beach at once. They visit a single beach. This is generic, but not sporadic.

Heyo! I can't help you out with sporadic or generic references since I've never heard the terms, but I found something useful about using "the." It's an grammar article about when and when to not use "the," and it'll answer a lot of your questions: www.edufind.com/english-grammar/definite-article/