These terrorists typically believe that their actions will cause other people to rally to their cause, eventually resulting in the downfall of whatever group or system they believe is the problem. This is partly why they often create manifestos.
The Oklahoma City bomber was motivated by anti-government ideology. He seemed to believe that his actions would eventually result in the judgment of government officials, such as those at the Bureau of Tobacco, Alcohol, and Firearms:
ATF, all you tyrannical people will swing in the wind one day for your treasonous actions against the Constitution of the United States. Remember the Nuremberg War Trials.
He may also have been motivated by white supremacy, since he had copies of the racist The Turner Diaries and seems to have been inspired by it.
The Christchurch shooter was motivated by white supremacist ideology and anti-Muslim beliefs. His goal was to start a race war, for instance in the US:
This conflict over the 2nd amendment and the attempted removal of firearms rights will ultimately result in a civil war that will eventually balkanize the US along political, cultural and, most importantly, racial lines,
He believed that several implausible political events would be triggered by his actions, such as an uprising of white inhabitants in Turkey.
This reasoning is not so different from that of more organized groups. Two of your examples, South Africa and Northern Ireland, are of successful terrorist groups, in the sense that the goal advocated by the group employing terrorist strategies succeeded, at least in part. But in reality, many terrorist groups toil in relative obscurity for years in the hopes that they'll get enough people to join them. In that sense, their expectations of success are not necessarily much more well-founded. Even a major terrorist group like al-Qaeda doesn't necessarily have good odds of success: they've neither driven the US from the Middle East nor installed a global caliphate, as they originally planned.