Since it's been more than a day and you still didn't get any answer I will try to formulate a response to the best of my capacities although I am no historian.

First you have to know that the UN project was Roosevelt idea and he was the one that pushed for it eventually giving it life. So it was the US that carried most of the process that founded the UN.

The goal of the UN, according to Roosevelt's vision, was to create an international community meeting place for nations to deal with their problems through consensus and discussion instead of conflict. This organization would thrive thanks to the leadership and guidance of the great powers of the post-WWII world, meaning the big Three (US, UK, USSR).

Since the UN was to begin as soon as the war was to end, the talks dealing with how the UN was to be constructed were lead during the Big Three meetings during the war, where they also talked about what to do with Germany and Japan after their ultimate deafeat.

The conference to remember is Yalta (February 1945) where the final permanent members composition of the Security Council were cemented. Obviously three seats were reserved for the UK, the US, and the USSR. Then there were long discussion about whether to include any more. Keep in mind that those discussions were ONLY conducted between the three which means that any additionnal country was to be proposed by one of them and be run through the other two. This is what happened for China and France.

So let's go first with China. This one was proposed and heavily championned by the US as Roosevelt was convinced of China's future role as a regionnal power after Japan's ultimate demise and its potential to stand as equal to the US. The UK cared very little for this choice while the Soviets had very mixed feelings about this. In the end the USSR gave in after the British got behind the US proposition to which they opposed their own which were the French.

The UK knew that it was exhausted from the war and wanted a second european power to counterbalance its decreasing stature with both the USSR and the US ever growing power. Plus, At that time France was already liberated and taken care of by De Gaulle and though he was hated, The British trusted him to try his best to keep an iron grip on the French Empire as the British projected to do the same for their despite the US and the USSR clear distaste of colonization. So the British pushed for France and the US though at first opposed relented since the UK had accepted China, and the USSR shrugged and went with it as long as both the other were okay with it.

That's thus how the UNSC was formed. Then the US also tried to include Brazil as a South American ally and power but both the UK and the USSR flat out refused.

So to put it in a nutshell here is a summary through a few bullet points: *UN inception came through the US with the cooperation of the UK and the USSR *(Only!) the Big Three decided who gets to seat with them during the war-time meetings. *China was added by the US and France by the UK, both can be considered as afterthoughts, especially France as is was not part of Roosevelt's plan but a concession *Brazil was proposed but got rejected

In the end it is also important to remember that when it was imagined, the UN was counting on the cooperation of the Big Three and as a result did not foresaw the US/USSR divide nor the British (and the French) Empire crumbling nor China's turn to communism which resulted in sour relations in the UNSC and its relative ineffectiveness all throughout the Cold War (by that I mean the abuse of veto and the clear East/West divide)

I hope that I was clear enough for you !

Edit: I'm sorry I forgot to put on my sources ! So here we go: *Act of Creation: The Founding of the United Nations by Stephen C. Schlesinger (very comprehensive if you are looking for more information I advice this one) *Charles De Gaulle's Memoirs Third Book (though these are most probably very biased)