Let's be real clear about the "separatists'" notion.
What happened in 2014 wasn't some popular uprising with a grassroots element that felt like the rightful president had to flee in a coup.
The popular uprising occurred in Kyiv. It was massive, ongoing, and lengthy, through Ukraine's bitterest cold. Eventually, after massacring his people didn't make it stop, Yanukovych was faced with prosecuting the kind of national annihilism Assad did in Syria to retain control of the country. He chose to flee instead.
Afterwards was a time of tumult. It wasn't clear who was in charge. No one had any popular mandate to rule.
The military had been purposefully gutted by Yanukovych in favor of his security forces. Corruption was rife (always has been, and frankly, almost certainly exists now even if muted).
In this turmoil Putin's first concern was Sevastopol, the historical Russian naval base, home of the Black Sea Fleet, and inarguably the single biggest military asset in all of Ukraine. He wanted to make sure he secured that in the face of a Western dominated Ukraine. So the green men (unmarked Russian soldiers) took over Crimea without a shot being fired, including several Ukrainian warships. The commanders on those vessels complained of no chain of command, no instructions on how to handle the current situation. The sailors on those ships were offered Russian citizenship, or were sent back to Ukraine in humiliation.
The referendum to annex Crimea wasn't as much of a joke as the recent one involving the four regions of Ukraine, but it was close. Same armed troops at the ballot box. They intimidated the local Tartars to stay away from the ballot box. This is in addition to Crimea being primarily Russian, mostly because Stalin round up the native Tartars and sent them to Siberia while migrating huge numbers of Russians to the area.
After that was secured in good order, Putin expanded into the Donbas, a region that has been a historical manufacturing base for Russia, including military vehicles in modern times. The green men systematically took over law enforcement, judicial buildings, and political centers of control practically overnight throughout the area. They were repelled in some places, but in town after town most civilians woke up with someone else in charge and a feeling that there wasn't much that could be done about it. New Russian appointed leaders announced their control of the regions.
The political leadership in Kyiv was a complete clusterfuck trying to respond to any of this. Eventually they did send the hollowed shell of a military they did possess down to recapture the regions, including thousands of volunteers, most notably what became known as the Azov battalion. Many of these volunteers were the foundational source of what Putin has described as the "Nazification" of Ukraine.
It's important to note, and I've done it on this thread before, but I wouldn't expect anyone to remember, and I sure wouldn't expect anyone to go back and read about it, but when Russians call elements of Ukraine Nazis, they don't mean it in the way westerners think about it. That's why it's idiotic to bring up Zelensky as a Jew as part of the response. Russians don't primarily associate the Nazis with the Holocaust the way westerners do. Heck, Russians have their own ambivalent historic relationship with Jews. Russians associate Nazis with Operation Barbarossa, and the whole sale massacre of Russians, the vast destruction wrought, the rapes, the terror campaigns, both incoming and in retreat.
Regarding Ukraine itself, Stephen Bandera is the central polarizing figure. For Ukrainians who associate with the west, he's a national hero, a wanna be liberator. For Russians he was a Nazi collaborator, who was responsible for atrocities against the Russian people.
It was under Bandera's banner that many of the volunteers headed east with the Ukrainian army to recapture the Donbas region, many of them part of the so called Azov Battalion. And, frankly, they were wiping the floor with the rag tag groups Russia had pulled together to take over the Donbas regions. Until the Russian army crossed the border and virtually wiped them out. Russia wasn't willing to openly invade Ukraine at that point (they probably should have given what has happened to them in the last 8 months). They took out the Ukrainian forces, and after that the front line stabilized, and would remain that way for the most part for 8 years.
But while there was always a more pro Russian sentiment in eastern Russia (Zelensky himself is a native Russian speaker, and favored a softer approach to Kremlin than his predecessor, Poroshenko), it was Russian machinations which created the breakaway Donbas region in the first place. Not some separatist movement. That part was fictional.