Short answers, pulled from the story.
The Union of South Africa came into existence on the 31st of May 1910, when the Cape Colony, Natal, the Transvaal, and the Orange River Colony unified into a single state. It existed as a British dominion and later a Commonwealth realm until the 31st of May 1961, when it became the Republic of South Africa.
Louis Botha was the first prime minister of the Union of South Africa. He had previously served as a Boer general during the Second Boer War and headed a coalition representing the white Afrikaner and English-speaking communities.
The transition resulted from a referendum held on the 5th of October 1960, driven primarily by Afrikaans-speaking voters and the National Party, which had long opposed ties to Britain. The referendum passed due to the larger Afrikaner electorate, and on the 31st of May 1961 the republic was proclaimed, with South Africa simultaneously leaving the Commonwealth of Nations.
After the outbreak of the First World War in 1914, Union forces occupied German South West Africa. Following the war, the League of Nations granted South Africa a Class C Mandate to administer the territory. The Union treated South West Africa as a de facto fifth province, and in 1949 passed a law giving the territory representation in the South African parliament.
The Statute of Westminster, passed by the British Parliament in December 1931, confirmed that the Union and other dominions were equal in status to the United Kingdom and that Westminster could no longer legislate on their behalf. South Africa then passed the Status of the Union Act in 1934, incorporating those provisions into South African law and removing any remaining British role in granting Royal Assent.
Roughly 28,000 people died in the concentration camps in which Afrikaner civilians from the Boer republics were interned during the Second Boer War. Britain used these camps as part of a scorched-earth strategy to deny guerrilla fighters civilian support, combined with a system of blockhouses to track down holdouts.