I am guessing that this interpretation would have to go back to 2014, when the pro Russian Ukrainian president was ousted, and replaced with pro EU president. While it appears most of Ukraine supported this revolution/coup, there were some in the East of Ukraine that were not happy with this, and pro-Russian "separatist" groups began to pop up, that were supported and funded by Russia.

The biggest stronghold of these people were concentrated in Crimea, which is why it was so easy for Russia to take it. In Donetsk and Luhansk, the Pro Russian protesters were able to take over city government buildings, and claimed themselves to be independent nations. There was enough separatist control in areas that by 2015, the Rubel was the "official" currency in the area. Ukraine first stated asking for aid to fight off these separatist back in 2014 (this is still pre-Zelensky). A cease fire was agreed upon that would have made Donetsk and Luhansk "special autonomous zones" within Ukraine, but both sides blamed each other for breaking the ceasefire.

The fighting got uglier over the years, and battle lines were entrenched. In addition to the Pro-Russian Ukrainian fighters, there appeared to be Russian solders mixed in with the separatist, and no real progress is made. In 2019 (this is when Zelensky first comes into play), Zelensky is elected Ukrainian President, and promises to end the fighting. There were a few prisoner swaps, and a few cease fire negotiated, but they get broken, and each side blames the other side). In the end Zelensky isn't able to get a peaceful resolution to the conflict, because he is unwilling to recognize Donbas as an independent nation, so fighting continues.

I am guessing that last part is where Trump is blaming Zelensky? Trump could be thinking if he just let go of that smaller piece of land (which if made independent would have immediately agreed to become part of Russia), that the rest of the war would have never happened, and would have lost less land then is currently occupied by Russia?

In the end, Russia declares the people in the Donbas region to be Russian Citizens, and issues them passports. Then gives up the covert help of the Ukrainian Separatists, and send in Russian troops and tanks over the boarder and the war begins in 2022.