Look at the subject. Does she like it? pairs with he, she, it, and singular nouns. Do they like it? pairs with I, you, we, they, and plurals. The same subject decides questions and negatives, so He doesn't but They don't.
Paired examples
Does the courier have my parcel? versus Do the couriers have my parcel? One courier takes does; several take do. Both messages ask the same thing, and only the subject changes.
She doesn't answer her phone. versus They don't answer their phone. The subject still decides the verb in negatives: she takes doesn't, while they takes don't. Doesn't and don't are the everyday choices, and their full forms does not and do not simply sound more formal, as in She does not answer her phone.
Where does Priya sit? versus Where do Priya and her colleagues sit? Priya is a third person singular subject like he, she or it, so she takes does; add colleagues and the subject becomes plural, taking do.
Doesn't he agree? shows does at work in a negative question too. Just as he doesn't agree takes does because he is third person singular, the negative question keeps the same verb, with he staying right beside it.
The I and you wrinkle
One wrinkle to watch: I is singular in meaning but takes do. You say I don't know, never I doesn't know, even though I refers to one person. You behaves the same way, singular or plural: You don't need a ticket works for one friend or a whole group.
This matters when a message shifts subject mid-thought. Does he agree? is safe, but once the subject becomes I, the verb shifts with it: Does he agree? I do think so. Reading each sentence separately keeps the subject clear.
So two people still take do, and you fits both a friend and a crowd. But I and a singular you both break the one-person-means-does pattern, and they break it every time.