Hi, so I'm not a qualified historian, but I'm going to try to answer this anyway. The Russian Empire did in fact push into the 'stans, and during the 19th centuary there was something of a power struggle in Central Asia between Britain and Russia known as the 'Great Game'. This largely resulted from British fears that Russia would try to invade India and Russian fears of British influence in Central Asia. This eventually resulted in Afghanistan becoming something of a buffer state between British India and Russian Central Asia, which is how the Wakhan corridor came to belong to Afghanistan. As far as the border further east goes, they did in fact take some outlying parts of China in 1858 and 1860 following the Second Opium War. In 1875 they defined all of Sakhalin as Russian, rather than Japanese territory, however in 1905 the Russo-Japanese war resulted in half of Sakhalin being ceeded to Japan. It's worth noting that the Russian imperial influence in China extended beyond just official borders. This was shown in the 1904-1905 Russo-Japanese war, where the land campaign was fought in Chinese Manchuria and Korea between Russia and Japan. In this case, Russia's interest was to assert dominace over Manchuria, particularly the warm water port of Port Arthur in Liaodong province and also to prevent or at least hinder Japanese control of Korea. To increase their influence over Manchuria, Russia helped build the Chinese Eastern Railway linking Port Arthur to the wider Trans-Siberian Railway. Ultimately, Russia lost both the lease of Port Arthur and half of the Chinese Eastern Railway because of the war. As far as Mongolia goes, I guess that it just wasn't considered valuable enough to make the effort to take it from Qing China.