It's all down to the fact that "chłopcy" is what's a masculine virile noun, in other words it describes a person, not an animal or an object. So while "chłopcy" and "buty" are both masculine, they have different adjectival endings because of this <sub>virility</sub>.
I hope this makes sense.
I'm quite new in Polish but I guess that it helps to differentiate singular ("nowy/nowa") and plural ("nowi/nowe") forms of adjectives. "NowY buty" will sound approximately like "two new shoe". "Nowy" can be only one "but". Adjectives don't have the same endings as nouns. It just a coincidence that plural noun ("buty") and male singular adjective ("nowy") have the same ending "Y". If I'm wrong, correct me please.
It isn't that way for the same reason this is wrong: "Cats likes milk." It isn't about matching the ending letter to be identical.
Most words that end in a consonant are masculine. You can also check them in a dictionary to make sure. The adjectives usually end in i or y.
So if it is masculine, you have:
- Nowy but, nowy kot, nowy chłopiec, nowy komputer
- Słodki chleb, słodki pączek, gorzki melon…
- But you also have some exceptions, like nowy mężczyzna
And most words that end in a are feminine and the adjective ends in a:
- Nowa wódka, piękna kobieta, słodka babeczka
- Some exceptions include mysz (Ta ładna mysz lubi ser)
And neuter words usually end in e, o or (rarely) um and the adjective usually ends in "e." Plural nouns of any gender almost always take the same adjectives, but there is an exception for that too.
- Nowe koty, nowe komputery, słodkie pączki, piękne kobiety…
But if it is a masculine personal noun, like "ludzie," it has a new ending using "nowi" or "nasi"
This one is still a bit new to me but I hope the others helped