During the interwar period, the Empire of Japan had two main expansion doctrines, the Northern Expansion Doctrine (Hokushin-ron) and the Southern Expansion Doctrine (Nanshin-ron). Japan had originally been more concerned with the ideas of Hokushin-ron and had clashed with the Soviet Union multiple times throughout the 30's and 40's in skirmishes that are known as the Soviet-Japanese Border Conflicts. 1939 was the point at which the Japanese halted their expansion north and decided to approach their expansionist ideas through Nanshin-ron. These were the battles of Khalkhyn Gol, in which Japan and the USSR and their puppets, Manchukuo and Mongolia respectively, clashed near the Khalkyn Gol River near the modern day border between Mongolia and Manchuria (PRC). The most decisive of these conflicts is known by the Japanese as the Nomonhan Incident in which a battle between the Japanese clashed with the Soviets for influence and control over the region. This battle lead to the defeat of the Japanese Sixth Army and their adoption of the Southern Expansion Doctrine. Had this battle been won by the Japanese however, it is very likely they would've continued their campaign into Siberia and Mongolia. This completely changes the course of WW2 as the Soviet Union was in my opinion the most major player in the war. For nearly two years, from the beginning of Operation Barbarossa to the Battle of Stalingrad, the Soviets were able to slow down the German War Machine and eventually lead the push to Berlin through sheer willpower and ability to outproduce and outnumber the German War Effort. Had the Japanese been able to invade Soviet-held Siberia, it would have created a two-front war for the USSR. Remember, the Germans only came withing 30 kilometers of Moscow, so if the Soviets had not been able to focus their entire populous and production ability towards repelling the Germans, it is likely they would not have been able to stop them from reaching Moscow. If this is happened, Japan could have easily negotiated the annexation of Siberia and then moved on to China and Indochina. Both of these military actions would be easy for the Japanese government to justify as one of the policies that both doctrines was that the goal of their expansionism was to "push Western powers from the regions." Japanese Siberia would have been able to provide a lot for Japan, mostly through industry. Had they conquered Siberia, the Japanese would have moved onto conquering Eastern China and Indochina, as well as Pacific island chains like they did in our timeline. Acquiring Siberia, East and South Asia, and the Pacific would have given Japan a massive population boost (today roughly 1.6 - 2 billion people) as well as a huge amount of land (I estimated about 8,909,447 sq. km. of land*). Domination over Asia and becoming a superpower would follow in the wake of this massive conquest of Asia. As stated before, the Japanese Invasion of Siberia probably would have given the Nazis the chance to conquer the USSR, so a alternate Cold War would likely occur between the Empire of Japan and The Third Reich. The Japanese planned to use Siberia more for industrial purposes rather than settlement (Siberia is really cold) and so to answer your question simply, the Japanese would have used Siberia mostly for industrial purposes, similar to the American use of the Midwest for more agricultural needs than for population settlement. Their fail to conquest Siberia really does change a lot in our timeline, so this was an interesting topic to try and figure out.

*The way I calculated the rough land area Japan would be annexing was by adding the rough land area of the east coast of China, as they did have written plans to directly annex this, the rough area of far-eastern Siberia, the land area of French Indochina, as they also planned to annex this, and finally the land area of modern day Indonesia. Although I left some parts out, like Taiwan and other pacific islands, this was a good rough estimate based of the alternate history question you proposed and actual Japanese post-war plans.