Ending "-yo" makes a sentence polite a bit and informal.
(it's kind of "해요체-Haeyoche")
1.아니(a-ni) - impolite and informal
2.아니요(a-ni-yo) - polite and informal
3.아뇨(a-nyo) = a contraction of "아니요"
As for "가위-바위-보",
/w/ sounds as in "가위", "바위" should be prounounced exactly but
when we are saying it fast, often the /w/ sounds are ignored because /이/ sound is easier to pronounce than /우/ sound. This happens in conversation.
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