you should look it up because it's interesting, but basically it was a compromise. Stalin claimed the UN was too dominated by Western oriented nations and demanded representation for every SSR (would amount to 16 votes, which was incredibly significant at the UN's founding). Instead the organization compromised on individual representation for Belarus and Ukraine along with the Soviets. I know amendments had to be made to the Ukrainian constitution for this, can't find information for Belarus but I presume the same was true. These amendments allowed Ukraine (and presumably Belarus) to act 'independently' as far as international law was concerned. However in every practice they had no independence in the UN, even despite Ukraine's appearance on the Security Council, they were just an extension of the Russians.
Why does it go Eritrea, Monaco, then Andorra