QW refers to such interrogatives as だれ(who), 何(what), いつ(when), どのN(which N), and どこ (where). This expression indicates that what is stated in the predicate is not limited to N1 or N2, but applies to everybody/everything/everywhere, etc. under the same category. When another particle is used with でも, it precedes でも(e.g., だれとでも[with anybody]).
(子どもでも大人でも)だれでも法律を守らなければなりません。
[Whether one is a child or an adult,] everybody is obliged to obey the law.
あの病人は、(肉でもやさいでも)何でも食べられます。
That ill person can eat anything [whether it be meat or vegetables].
スーパーマーケットは(東京にでもおおさかにでも)どこにでもあります。
Supermarkets are everywhere [whether it be in Tokyo or in Osaka].