Hey there, South African lover of history here, let me be the first to hopefully provide you a satisfactory answer to your question

The first thing you need to understand about South Africa, especially the South Africa of the early 20th century is that "white" is a very broad umbrella term, encompassing groups which didn't feel much affinity with eachother and whose political differences are vitally important to understand the. Now why focus on the whites specifically in this answer? Well the formation of the modern South African state in the final decades of the 19th century and the first few decades of the 20th occurred behind the backdrop of the height of Victorian era scientific racism This was a time when in the west black Africans where considered barely human if at all and thus where where denied any real representation until they had to fight and campaign for it from the 1950's onward. Instead the political arena in which Apartheid came about was dominated by the competition between two groups, the Afrikaners (Or Boers) and the ethnically British people of the country.

Now since you mention you aren't too clued up on South Africa's history, let me just quickly digress and explain the origins of each group and the class differences among them which (to my mind) helped contribute to Apartheid.

The Afrikaners

The Afrikaners are the descendants of mostly Germanic Europeans who settled in the country after what is today Cape Town became a outpost of the Dutch East India Trading Company. The irony is that Jan van Riebeeck, the expeditions leader was under strict orders not too let the colony grow too large in size as its intention was merely too serve as a refreshment station for ships en route to the Far East, but the south South African climate tends toward the semi arid and drought prone and keeping up with the agricultural demand meant farmers had to venture further and further away from the coast. Within a few generations many of the migrant farmer families had begun venturing so far inland that the authorities in the cape colony had effectively lost authority over them, only bothering to intervene on the frontier if the wars over cattle and grazing land broke out between settlers and the khoisan natives. Many of these people where the descendants of social outcasts from European society like fugitives and religious refugees fleeing persecution. and this hatred for authority and self reliance that the frontier lifestyle required soon fostered a separate identity among these people that differed from the more continentally minded people inhabiting Cape Town itself. By the 18th century we have literary references of the Cape authorities referring to the inland farmers as "africaander" or "Africans". The

The British The second group, the British became a permanent political feature of the country after the battle of blauwbergin 1806 during the Napoleonic wars. The aftermath of the battle led the Dutch to cede the colony to the British and the tip of Africa essentially became a sphere of British influence. The British mostly preserved the status quo, only seeing the cape colony as a useful staging ground to control the sea lanes between Europe and Asia. The colony had by know grown to infringe upon the Nguni groups such as the AmaXhosa who lived along the colonies eastern frontier and the British began to establish mission stations and forts, along with some disastrous attempts at establishing British farmer communities such as the founding of Port Elizabeth in 1820, where q few thousand British settlers with almost zero experience in agriculture where unceremoniously dumped in Algoa Bay. Many of these newer immigrants did quite well for themselves, and the "nation of shopkeepers" as Napoleon once referred to them soon began to fill the niche between the farmers of the interior and the foreign markets that sought after South African export goods like wool and wine (The mines would only come into play at the end of the century) Now the big change which arguably set the whole modern history of the country in motion came in the 1830's when the British where conducting their global anti slavery campaign and as part of it began introducing legislation which tried to stop the Trekboers from using their indentured servants as labour without pay. and required such things as Though perfectly reasonable from the authorities perspective it struck the Afrikaners as a bizarre affront. as many of them had been living with these native and mixed race families for generations, often in a sort of quasi familial (though very unequal) relationship. Another gripe was simply the fact that the cape colony was governed by Britons, whom the Afrikaners on the frontier felt weren't doing enough to safeguard them from the regular Xhosa incursions across the border and where ignoring their concerns regarding the land shortages that the recent influx of British settlers and droughts caused. This and a host of other differences eventually prompted what would later become known as the Great Trek, an organised mass migration by a few Afrikaner communities to the north to create new republics for themselves free of any foreign influence. Now there were numerous waves of these, and enough events to fill many books about, but basically understand that this was a watershed moment in Afrikaner history which along with the Boer war(which i will discuss below) would form the two pillars of 20th century Afrikaner nationalism. Similar to the American Manifest destiny, the Afrikaners believed then in an old testament like covenant between them and god which granted them divine right to the lands they entered. this belief mostly came about after their victory at the battle of blood river, where a traveling column of Boer settlers managed to stave of an attacking Zulu army many times their size. More info here

(Hey there, if youre reading this know that i am still editing this comment and will complete it shortly)