I know Native Americans crossed over from Russia, but how did Russians (or whichever nation was in the Eastern most part of present day Russia) not sail to Alaska in the 4000+ years following? Even the Pacific Islanders managed to make it to Hawaii, so I can’t fathom Russia or Japan failing to explore to the northeast.
Siberia wasn't truly "Russian" until the Mid 20th century. For most of human history, it had zero or very few Russian inhabitants, and it wasn't until mass deportations and forced cultural assimilation by the Soviet Union that the region became Russified. I've written on that here.
People of the Eastern Siberian coast (Chukotka and Kamchatka peninsulas) were well aware of the existence of North America. We have substantial evidence of trade, migration, and even conflict, between peoples on either side of the Bering Strait, for the past several millenia.
Wind and ocean currents may occasionally blow Japanese fishing vessels northeastward, which led to very rare--but very real--shipwrecks of Japanese vessels on the coast of North America. While its true that the people of Japan itself were not aware of the existence of North America, a very small and very unlucky minority of Japanese fishermen likely learned about it 'the hard way' centuries before Columbus.
I've written a few answers on this subject before.
How we know people migrated from Eastern Siberia to North America
Ancient Contact between East Asian and Western North American Peoples
How many times were the Americas "discovered" before Columbus?
Why China, Korea, and Japan, did not know about the Americas