I'm curious, before russia took outer manchuria from china and took chinas access to the sea of japan. how much of the russian far east did china explore and settle?.
As /u/EnclavedMicrostate states, there were few long-term Chinese settlements in what is now the Russian Far East. There were, however, times when expansionist dynasties throughout Chinese history were interested in the fringes of Manchuria (especially for pelts and falcons), and established links to the region politically, commercially, and militarily.
Tang dynasty records dating from the 7th century tells of the country of Liugui (流鬼) far to the north that Japanese and Russian researchers identify as Sakhalin Island off the East Siberian coast. In 640, the "Hairy People" of Liugui (identified as the proto-Nivkh Okhotsk culture) sent an embassy to the Chinese court with gifts of sable fur, and the Tang court, in the tradition of the tributary system, conferred legitimacy to Liugui by granting its "prince" an official title.
Evidence of a Chinese, or rather, a Sinified presence in the far edge of Manchuria can be found in Tyr, Russia, where cossacks of the 17th century found a pagoda standing on a cliff near where the Amur River dumps into the ocean. The pagoda was demolished in the 19th century, but a study of the base structures reveals that the pagoda was likely part of a Buddhist complex erected prior to the Mongol conquests, either by the Khitan Liao or the Jurchen Jin, who were both partially Sinified dynasties that had a presence in the area.
When the Mongols conquered the Jurchen Jin, they maintained an outpost at Tyr (then called Nurgan). Here they clashed with the indigenous Nivkh and Udege peoples and forced their surrender. The Nivkh, who were used to travelling across the frozen sea to-and-fro Sakhalin, provided the Mongols with furs that were in vogue among the Mongol-Chinese elites at the time, but they also expected protection from the Mongols, as they were under attack from the Ainu, who were expanding from Hokkaido to Sakhalin. The Mongols fought a protracted but intermittent war with the Ainu from 1264 to 1309, and at one point even brought Han settlers from the Southern Song (which the Mongols defeated in 1279) to Manchuria to support this effort. These Han settlements were withdrawn when the Mongol prince Nayan rebelled against the Kublai Khan. In 1309, the Ainu agreed to submit to the Mongols, and from then on they would "present tribute" to Mongol outposts on Sakhalin.
While the actual extent of Mongol control on Sakhalin is unclear, Japanese archaeologists found ramparts at the southern tip of Sakhalin that correspond to forts of a continental (Chinese) style dating to the 13th century, at the time of the Mongol-Ainu war. It is theorized that this specific outpost (if this is actually a Mongol outpost) was built to keep watch on the strait to prevent further raids from Hokkaido.
The Mongols were replaced by the Ming dynasty, who inherited the abandoned Mongol outposts in the lower Amur region. The Yongle Emperor, who famously sent the eunuch Zheng He to sail to the Indian Ocean, also sent an expedition under the eunuch Yishiha to reestablish Chinese authority in the Amur region and Sakhalin in 1411. The Nivkhs in the region were organized into an administrative district at Nurgan under a native chieftain (tusi), and were expected to send tributes to the Ming court. Yishiha, for his part, made a show of force by sailing to Nurgan in 25 big ships, installed troops in the area, and distributed clothes among the natives in Sakhalin. Yishiha came to the area again in 1432 to quell a local rebellion, this time bringing 2000 soldiers in 50 ships. Steles were erected for the occasion, and they are now part of the collection of a museum in Vladivostok.
After the death of the expansionist Yongle emperor and the debacle of Tumu in 1449, where the emperor was captured by the Mongols, the Ming dynasty turned inward and left the Amur outposts to their own devices. The Nivkh and the Sakhalin Ainu presented tribute until the late 15th century.
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The regions seized by Russia in 1858 and 1860 had never been part of 'China' as a geographical entity. While they had been under de jure Qing rule since 1689, with a clearly delineated border, these were areas inhabited principally by non-Manchu tribes that held relatively loose connections to the Qing core. See this answer for context. It is worth pointing out that the Qing were not, strictly speaking, 'China' – rather, the Qing were a broader empire that controlled regions of Inner Asia that were recognised as distinct from its core dominions in China. 'Inner' Manchuria, that is the settled region in the southern part from which the Manchus emerged, was already relatively far-off as far as Chinese interests were concerned, although its western portion, the Liaodong region, had seen two periods of direct Chinese rule, once from the late 4th century BCE to the early 4th century CE, and later from the late 14th to early 17th centuries. But the region beyond this seems to have generally been poorly known. While we do have some ethnographic record, there was no long-term settlement. Even the Manchus did not make much effort to penetrate into Siberia. See this answer.