It's very complicated.

As I understand it, the origins of the Rus are not entirely factually established and there is some debate. The popular understanding is that germanic traders, likely from sweden, established a trade network on the rivers that drained into the baltic sea. Kyiv (kiev) was a major hub of this trade between the germanic swedes, and the native slavs. People of "viking cultures" such as the kievan rus were referred to as varangians by the greeks. Over time, these norse/varangians folks adopted local languages and a blending of cultures occured. A guy named Rurik established a dynasty in novgorod in the mid 9th century, during the viking era, and his descendants held a dominance over the local slavs, and it expanded to include finno-ugric people in the north.

This is how we got proto russia, called the kingdom of the kievan rus, centered on novgorod. Ukraine was the southern extent of this kingdom

Around the middle of the 13th century, the Mongol Empire included most of eastern europe and ukraine. As the empire fragmented, a local portion called the Golden Horde held sway over most of the Pontic Steppe, including Ukraine. A number of khanates splintered and warred with eachother. Over time. It's complicated.

In the 16th century, this now mostly turkic kingdom was pinched between warring against the polish, lithuanians, and a competing khanate in the eastern part of the region. The Grand Duchy of Lithuanian and Poland controlled most of ukraine as part of their territory, leading up to and after that conflict. Eastern Ukraine was largely under the control of the Crimean Khanate.

Okay. Up to this point it's 1502, eastern ukraine is run by crimean cossacks, western ukriane is between the polish and the lithuanians who later in the century unified into the Polish-Lithuanian Commonwealth, I'll refer to this as the PLC to save time. Lets change gears and talk about russia and the mongol empire.

Between the 9th and 15th century, The Kievan Rus culture, this blending of slavs, norse, and finns, had spread around the territory of the eastern slavs and adopted Eastern Orthodox, and a mainly Slavic language. Kievan-Rus as a political entity and a country was conquered in the 13th century by the mongols. By the end of the 15th century, the Rus in the northern part of the empire, centered on Moscow, were sick of mongol hegemony and the local leaders put together enough to essentially rebel and formed the Grand Duchy of Moscow in 1480.

The Grand Duchy of Moscow basically worked to reunify culturally Rus peoples into one kingdom either by conquest or politics, and became the Tsardom of Russia by 1547. This pretty much cemented the foundation of what we recognize as Russia today. In 1721, Peter 1 established the Russian Empire, which expanded quickly and lasted until the revolution of 1917.

Okay, so. Back to Ukraine in the 16th century. Ukraine was never really a distinct political entity. There were major trade hubs there during the dark ages where the russian culture first sort of germinated, eventually becoming part of a larger Kievan Rus kingdom for a few centuries. Then ukraine came under centuries of mongol rule, then PLC rule, while the modernish Russian Identity developed and cemented further noth and east around Moscow and the rebellion against the Mongols.

Around the 17th century, PLC and and the Tsardom of Russian went to war, essentially for control of Ukrainian terrirtory. Cossacks under nominal PLC authority rebelled, likely aided and encouraged by Russia. Then russians invaded what is now ukraine. They also overthrew the cossacks of crimea somewhere in there. Sweden also invaded PLC at the same time. After a decade and change of war, PLC and Russia agreed to stop, and ukraine became a client territory of Tsarist Russia.

It remained under tsarist russian authority until 1917 when they won their own independence before becoming quickly subsumed by the bolsheviks. Thence unto SSR, thence unto Modern Ukraine.

TLDR. Protorussia formed in the dark ages. Mongols tore up Proto Russia, the now-ukrainan bits got eaten by Poland. A while later, the Moscow Adjacent bits kicked the mongols out, then beat up poland and took ukraine for themselves. Then a lot of bad things happened and ukraine was able to become a politically distinct thingy mabob.