How do totalitarian states/empires usually fall? Are there cases or plausible where these states fall without it necessarily being invaded or face a rebellion? When i say fall, i mean the move from totalitarian to at least ''more'' democracy. For example, in the case of north-Korea which option or options are viable to happen.
Totalitarian regimes generally end in two ways:
By the sword: either a foreign invader or a domestic revolt or both forced an end to the regime.
By the pen: the ruling regime peacefully transitions itself into something less totalitarian.
Examples of forced changes includes Nazi Germany and Fascist Italy. This doesn't always need to be a prolonged bloody conflict, though: the Soviet Union quickly crumbled after first the Baltic Republics and then the Russian SFSR effectively rebelled against central authority. Sometimes a coup removed the totalitarian leader and reintroducd democracy, such as in the United States of Brazil and its former colonial homeland, the Portuguese Second Republic.
However, peaceful transitions absolutely do happen as well. Francoist Spain rapidly became a democratic constitutional monarchy after his death when Franco's chosen successor turned out to be a secret liberal. The path for Myanmar was a lot more fraught with setbacks: after elections were held in 1992, the military junta couped its own leader to hang on to power. But concrete liberalisation eventually took place after 2011.
Not all totalitarian regimes become liberal democracies in this way, though. The aforementioned Portugese Second Republic was ended by a coup in 1974; however, the regime engaged in limited liberalisation long before its fall. Depending on how strictly you define totalitarianism, it could fit either category. Likewise, totalitarianism in Communist China is said to have ended with Mao Zedonh 1976. But while subsequent reforms under Deng Xiaoping have greatly loosened socioeconomic controls, China remains indisputably an authoritarian state.
Perhaps the better way to phrase this is that - after WW2, anyway - the most common way for totalitarian regimes to go is by succumbing to a combination of domestic discontent and external pressures. Those who saw the writing on the wall and proactively reformed the country could manage a peaceful transition; those reacting too slowly had the changes forced upon them.
Lastly, all of these are viable possibilities for North Korea, although none of them seem particularly likely currently.
When authoritarian regimes fall or 'transition' into something less authoritarian it is typically due to either external forces (invasion or threat of invasion) or economic stress.
There are of course many examples of regimes falling due to an actual invasion, but there is one example of an absolute monarchy (Kuwait) which became a constitutional monarchy in the face of a threatened invasion from Iraq, which refused to recognize Kuwait's independence from the United Kingdom in 1961. This was the height of the Cold War globally and the heyday of Nasserist revolutionary republicanism in the Arab region specifically. The ruling family came to the conclusion that a constitutional system would shore up its legitimacy both at home and abroad in the face of Iraqi claims. This lasted for over 20 years before the ruling family managed to dispense with the constitution and re-establish absolute rule in the mid-80s. The constitution was only restored in the early 1990s, due this time to an actual invasion and occupation by Iraq in 1990-91.
I mentioned Kuwait because it is an interesting case that is not well known. More often, however, authoritarian regimes fall or transition when the economic basis for their rule is gone or ceases to grow. Most authoritarian regimes today are 'rentier states'. They are able to finance themselves from easy foreign sources such as the sale of natural resources (e.g. Arab Gulf monarchies) or outright subsidization by another country (e.g. North Korea by China, Belarus by Russia and a number of Arab countries by their Gulf neighbors). China is a unique case in that its economic growth is based on actual production, but whether the regime can survive in its current form when that growth slows down will be interesting to see.