United Nations- UN and its Mandate

"UN cannot have a mandate of its own"- sir please elaborate.

UN is a voluntary group of nations that have come together for common good, mutual assistance, security and development. The mandate of the UN is prescribed by the member countries. In an ideal situation, all countries agree towards a single goal and the capable ones help those in need, to achieve that goal. In reality, every country weighs the pros and cons of any resolution or a plan of action through the prism of self interest. Food for the goose is NOT food for the gander here! So, there are stark divisions amongst the countries and generally the stronger ones get their way through by cobbling up a majority.
 

It is that shifting majority that brings about the direction of the UN. Today's coalition for a resolution is not a guarantee for tomorrow's.

 However, when a majority is obtained, it gets the force of law and becomes the will of the nations. Remember, it is a voluntary group, so one abides by such resolutions. However, when some do not get that majority to proceed, they form their own coalition with a smaller group and go ahead. The action of such a group does not have the sanction of the majority and therefore lacks the sanctity and is not binding on all nations. Technically, this action is in violation of the collective principles, but UN can't take action here as it lacks the direction of the majority, who decide to not punish, not participate and ignore owing to their own self interest and the strength of the coalition of the willing. That is why it is said that the mandate of UN flows from the member countries.

Final question sir- as you said the membership to the UN is voluntary, hence its assumed that if one can't get majority and forms a coalition outside of the UN- that the 'mandate' of UN as for the current resolution at hand- is to refrain and not go against it.

Following the same, there should be a framework to 'impose' its 'resolution' on refraining such action, on its own member states? Because they're the one found in minority are UN members themselves.

Or should I answer my question saying that- the majority (say Russia, acting as a UN member state) wouldn't want to further spoil relationship on a bilateral level by enforcing sanctions (again, on behalf of UN or the majority coalition within UN) or deterrance in other forms- against USA?

 There is no such mandate to not go against it. Members who do not form part of the coalition, if they come together and get a majority, then the will of the UN is to oppose the action of the coalition of the willing. But countries chose not to press for such a decision for their own reasons. Spoiling relations is only one aspect that is considered.
 It is always a game of numbers. The more number on your side, the stronger you are. There of countries, by force of their economic and military strength and veto power owing to UNSC membership exhibit power and thwart a majority from being formed against it.