If you're asking why the singular German sentence translates into a plural in English - that's just how English works for this word.
✂ die Schere (singular) = "the scissors" or "the pair of scissors"
✂✂ die Scheren (plural) = "the scissors" or "the pairs of scissors"
In this way, it works like "the pants". There is no such thing as "one pant" or "one scissor". Singular would be "one pair of pants" or "one pair of scissors". You use the matching verb forms for the plural, too, like "were/are" etc. instead of "was/is".
I beg to differ. I'm a native English speaker and I would always use "scissors" in the singular. If I were asking someone to pass me the scissors, I would say "can you give it to me" not "can you give them to me." (Webster says that it's a plural noun but sometimes singular in construction.)
Which of these sounds better to you?
"The scissors need sharpening" (plural verb form)
"The scissors needs sharpening" (singular verb form)
Gotta say, I still prefer the plural verb form.
Unless you use "the pair of scissors", in which case the verb is based on the singular "pair": "The pair of scissors needs sharpening".
I'm a native English speaker and would always say "can you give them to me", whether it's for scissors or trousers (nobody says trousers any more except me) or underpants or shears.
If it was a box of matches you might say "give it to me, give me the match box" or just "give it to me" if you were thinking about the box, and say "give them to me, give me the matches" or just "give them to me" if you were thinking about the matches, but they'd still usually give you the whole box.
Would you also say "The scissors is on the desk" instead of "The scissors are on the desk"? I'd think not.
I do see why it would be logical to use "it" instead of "they" as a pronominal reference. But it would also be odd--or at least inconsistent--to treat it grammatically as plural in subject-verb agreement for the fully-spelled-out lexical item, but as singular in the pronoun form.
Although it sounds similar, that's not correct. "Shears" in English are like a large pair of modified scissors, often for a specific purpose. Out of context, you would need to use the specific word in German, such as Metallschere, Heckenschere, Gartenschere or at least große Schere.