Why would Russia have discovered North America? In 1492, when Columbus reached the Americas, the future Russia did not even reach the Ural Mountains, let alone the Pacific Ocean. Russian rule over Siberia would be a long process that took several centuries of expansion and colonization not altogether different from western European colonization of the Americas. They wouldn’t get east of the Urals until 1580. Prior to that, Siberia was home to numerous indigenous peoples, and several of them - namely the Chukchi, Yup’ik, and others living at the easternmost edge of Eurasia - were certainly aware of North America. In fact, the Yup’ik lived and continue to live on both sides of the Bering Strait. But for the most part, indigenous Siberians were relatively isolated themselves, and any contact with nearby East Asian empires, let alone Europe, was practically nonexistent. In a way Chukotka and Kamchatka were closer to being western islands of North America than peninsulas of Asia.

Now, as to the British. There are actually theories that British sailors reached the Americas before Columbus, though they’re not really backed by evidence (and some, especially Celtic contact theories, have strong racist undertones). But the Atlantic is a huge ocean, and before Columbus crossing it was not really considered useful or feasible. The Viking expeditions were (somewhat) known, but they didn’t speak of a new continent, just distant and largely useless lands. And while it was established that the earth was round, the prevailing assumption was that crossing to China via the Atlantic was too long of a journey to work. It’s really not surprising, then, that the first attempted crossings came out of Spain and Portugal, since in the 15th century these were rapidly rising maritime powers. Portugal had begun skirting the coast of Africa and reached India by rounding Africa not long before Columbus sailed; and Spain, before conquering the Americas, took over and incorporated the Canary Islands in the north Atlantic, giving them the best possible launching point to sail west. The British weren’t engaged in this to the extent of the Spanish and Portuguese. But after the initial Spanish discoveries they didn’t lag far behind, either.

Now as to Japan and China: East Asia is not my strong suit, but I can hopefully provide some answers. You assert in your question that empires “love to do” colonizing. While true of many later European empires, I’d question the accuracy of this for most historical empires. Empires love to expand, but expansion is not the same thing as colonization. And as for China, they certainly didn’t need to colonize. It would not be an exaggeration to suggest that 15th-century China had most everything it needed within its borders. And if they did need something from beyond, they were surrounded by plentiful land they knew to be full of people on three sides; why would they bother with a vast and unknown ocean? It’s not that they didn’t have the capacity, but having the ability to do something doesn’t mean that you want to do it. Chinese fleets sailed to the eastern shores of the Africa, but this was a known course charting waters and coastlines abounding in people and trade goods. The North Pacific is barren water, and the coastline north of China is and has always been sparsely populated and uninteresting to Chinese rulers. Why would China need to sail across the Pacific? It was unknown, and there weren’t any possible benefits beyond that couldn’t be readily - and more safely - found at home or nearby. I know less about Japan, but they were similarly more oriented towards the known lands to the west than the unknown waters to the east. After all, the west was where the people were.

There’s another theory that might help to explain why Portugal and Spain were first, rather than China. Europe in the 15th century was composed of numerous small, bickering polities, with only a limited amount of wealth inside their borders. To gain wealth and power, it was much more likely that these small states would look beyond to distant - but more easily conquerable - lands, rather than face off against their easily-matched neighbors in the hope of gaining a small amount of land or resources. China, on the other hand, was a massive empire, with abundant natural resources inside its own borders. If one understands finding and exploiting more resources as a powerful motive for colonization, it makes sense that Spain and Portugal would be more interested in colonizing than China. Again, this is only one theory, but it helps to explain why Spain and Portugal were more likely to “discover” the Americas than one might expect.