It's is because गर्मी is feminine. It is not connected to the "they" because the को in the sentence (उन्हें = उनको) blocks the subject and makes the verb match the object
One boy go
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Why do they use लगती with "they"?
Based off of a previous post it seems to be because the लगती is referring to the ठंड which is feminine.
उनको गर्मी नहीं लगती है । Is this right? It counted it wrong since I said उनको instead of उन्हें