I wanted to ask, is it possible that Germany is using some Russian/Soviet technology in this story?
Hypothetically. OTL I've seen very little about Germany using Soviet factory production or military technology. They did make some relatively small use of French or Belgian techniques/production, and Italian after September 1943. This does not seem to have been well organized.
I wondered, for example, with the Luftwaffe Ilyushin Il-2, if the Germans had seized some of the factories where the aircraft were manufactured after the victory in the East. And they have access to material, labor, why not try to produce something that will benefit them. Would the Luftwaffe even be interested in such an aircraft?
Are there any Soviet weapons systems that the Wehrmacht would really like badly enough to begin mass production of?
Really tough to say. I've seen next to nothing in my last forty years of casual reading. The army did salvage a fair number of 76.2mm caliber cannon, the Zis -3 model IIRC. Trucks salvaged as field expedient, and there were some limited numbers of tanks salvage. But, I know nothing of salvage of actual weapons production OTL. About all I can really say is OTL German use of foreign industry in occupied Europe was a bit haphazard. ie: they requisitioned 30%+ of French railway locomotives and other rolling stock by the end of 1943. Which interfered with operations of French industry, agricultural production, and supplying OB West in 1943-44. Also a number of French factory tools, the large industrial machine tools, were carried off back to Germany. But, I don't know if much of that was done OTL in the east. The Luftwaffe has in 1942 initiated a batch of new aircraft development projects. Those might seem more productive than salvaging old Soviet production ?
My best guess Much of it would be up to the whim of German business who are interested in developing the new territories.
Another question, what is the relationship between Germany and Finland now that both countries have a land connection. The Finns had big problems with the lack of material, for example, they lacked locomotives and many other equipment. In OTL Keitel said that shipping by sea must be limited, now such an excuse is no longer credible. Will Germany continue to support Finland economically and militarily, or will it end this support because Finland no longer needs it so much?
Foreign aid to Germanys allies seems to have been at the whim of Hitler, and what benefitted interested parties among the nazi leaders. Maybe the Finns can obtain some of those French or Belgian railway locomotives and freight wagons? There is not much precedence here, I'd guess that German larger would be different in this situation than during the eastern war.
I also wonder what state Leningrad is in now, can the port be used, or is it completely destroyed and the Kriegsmarine can no longer use it? If the fighting in the Baltic Sea is over, it will facilitate the training program of the submarine fleet, and the Germans will no longer have to create a convoy system for their cargo ships that import raw materials from Sweden and Finland. Such a decision would increase shipping turnover and secure more raw materials for German industry.
Yep, there's a lot of long term benefit here. Im not 100% sure, but believe the intent was to develop the Estonian ports. Lenningrad was to be razed like a lot of other Soviet cities. But, I'd not predict that as a given. There are a lot of variables & incentives to retaining what survives of of the Russian and Ukrainian cities and towns.
Are the Germans in control of Tikhvin? If so, then they have access to large reserves of bauxite, which could or should alleviate their lack of aluminum...
The Tikhvin deposits and refining facilities are well within the area the Germans would occupy. How long it takes to get appreciable quantities depends on how much destruction of the refinery, the railways, and the electric production. The modern post 1920s Soviet built refineries were dependent of electric power. Both the generation and the refinery equipment would time consuming to rebuild. The initial project goal for the Soviet modernization was 20,000 tons of refined Aluminum annually. What the German might have reach for or would have been able to reach in 2-3 years I can't guess. Ellis in table 49 of 'Brute Force' places Axis Aluminum production in 1942 as 420,000 metric tons and 432,000 in 1943. Soviet production for those two years was 51,700 & 62,300 metric tons.