Hello! Recently, I have been very interested in the histories of Russia, Ukraine, and Belarus. I have read and watched a lot about the medieval state of Kyivan Rus, and I have found some very starkly different interpretations of it’s history. I would also like to mention that I know that was there was no Kievan/Kyivan Rus, and it was a termed coined at a later point in time with it originally just being called Rus. I call it that because that is what it is typically called colloquially. On the one hand, many Russians will assert that it was the predecessor of Russia, and Ukrainians and Belarusians are just subjects of the “Great Rus’ans”. Futthermore, they often say that Ukrainians and Belarusians have made up nations. While on the other hand, many Ukrainians call this historical and cultural appropriation and say that they are the true successors of the state and that Russians aren’t even Slavs and are a strange mix of the Mongols and Finno-Ugrics that made up their language and have tried to fabricate a claim onto being the heirs of the medieval slavic state. I would love what accredited historians have to say about this. I believe the truth of this topic is something in between as is the case with many polarized topics. However, I haven’t seen the facts about the history of a nation be so seriously disputed whenever I have researched the history of a nation.
While the more fantastical claims to the heritage of Rus' fall outside the academy (at least among contemporary scholars; certainly Soviet historians claimed Kiev/Kyiv readily--the irony of velikorusskij shovinizm notwithstanding), indeed there are still quite a number of interpretations.
For example, Franklin and Widdis' edited volume National Identity in Russian Culture, now somewhat out of date, makes some rather arresting claims transposing Russian continuity onto Rus' from otherwise strong historians. By uncritically incorporating and cementing nineteenth-century and post-Stalin Soviet histories into modern historiography, these authors do an immense disservice in presenting Russian antecedents as Russian themselves. For example, scholars from Rus' are presented as belonging to the "Russian cultural discourse"; that simply is not true. Imagine a future state merger between France and Germany: Framany. Should the future Framansch Historikien subsume these antecedent states, with a different national identity, worldview, and language, with the current "national discourse"? Does Napoleon become a Framansch emperor?
How seriously you consider the elision between Rus' and Russian is sort of up to you, but certainly that distinction must be made. The East Slavs that composed in part Rus' did come to comprise Russia as they do Ukraine and Belarus; Ukrainian, Russian, Rusyn, and Belarusian are distinct East Slavic languages, and in the succeeding centuries have developed unique national, linguistic, and regional identities.
The claim that "Ukrainians and Belarusians have made up nations" is, I suppose, true when referring to Rus', if we extend it to merge all Eastern Slavs more broadly (that is, just as there were not 'Ukrainians' there were not 'Russians'). As far as their modern states go, all can reasonably be viewed as inheriting Rus', but not with Russia as Great Rus' and other East Slavs as attendant states or malorusskiy. (For another example, Paul Hollingsworth's introduction to his translated Hagiography of Kievan Rusʹ has advertising copy of 'among the finest products of early Ukrainian literature...').
Pelenski's The Contest for the Legacy of Kievan Rus', an anthology, addresses exactly your question. Pelenski arguably swings the other way, with some whitewashing of pre-Ukrainian regions at the expense of what would become Muscovy, and focuses more on the political and geographical distinctions (that is, the Dnieper river flows chiefly through Ukraine and Belarus). I recommend looking into texts on Rus' itself; Christian Raffensperger's Reimagining Europe is not without its problems, but is a fine introduction to some of the contemporary debates on Rus'.
While the legacy has been fought over, and will continue to be, Paul Bushkovitch, an eminent Slavic historian, decided in a review of Pelenski that the question was "meaningless". Bushkovitch's rather glib assessment highlights some of the inanity in laying singular claim to a broad grouping of feudal states that crested more than a millennium ago; there's no shortage of historical victories and declines in the interviewing centuries.