You have a multi-faceted question here which is great, these are all important topics to talk about when talking about these types of things. It seems to me that you’re referencing Vikings and Mongols just as examples; however, I can’t really comment on Vikings for you but I can do the Mongols and if there are other Steppe Empires that you’re curious about such as the Seljuk Turks for example, I can comment on that as well in case you were just using the Mongols as an example rather than an actual specific.

For logistics and military, I wrote a post in the past about that and you can check that out here.

Taxation

Taxation is a whole different topic and there were different ways of taxing people in this large empire especially since it spanned across multiple types of civilization. For example, you wouldn’t tax nomads the same way you would tax sedentary people. According to the Secret History of the Mongols, Genghis Khan asked the Kereyit tribe to pay him a certain amount of taxes based on their flocks and herds. Since nomadic society was essentially based on their flocks and herds historians typically agree that this was a sort of levy type tax, but it seems rather strange that they (the Mongols) would mention in their Secret History of this one time that Genghis Khan asked one of the tribes for payment of their ‘tax.’ This would point to the possibility that at least during Genghis Khan’s rule, there wasn’t a set-in stone way. Remember that the other steppe nomadic people were part of the Mongol army, so that could have something to do with it as well.

The other type of taxation they would impose would be a poll-tax. This was for the sedentary peoples and it was based on their census. It is though to have started during the reign of the Great Khan Ogedei; however, we know for sure the Great Khan Mongke imposed it as we have a number of complaints from the Islamic civilizations in particular in regards to this tax. One of the big complaints is again, what seems to be a sporadic decision by the Mongols to extort money out of their subjects. Though this is something that could have been seen in this point of view from the Muslims as now, a Muslim peasant had a “Mongol” tax on top of his normal Islamic taxation. If the Shahs of Khwarezem weren’t popular before, I’m sure they would have been after their demise. I should mention that as time went on the Mongols will allow the Persian bureaucracy to run the regions, so that’s a positive I suppose.

And lastly, the trade tax. As you would imagine simply by looking at a map of the Mongol Empire, they had control of the Silk Road. That means that all profiting through the trade routes were taxed by the Mongols at a certain rate.

Overall you can say that aside from a few examples, Mongol taxation could be viewed from the perspective of a conqueror and it had 2 jobs, 1: to enrich the Mongols and 2: to oppress their subjects.

Administration

This is a tricky one, and also one that historians still like to go back and forth about. Supposedly, at the Great Mongol Kurultai of 1206, Temujin was given the title Genghis Khan and enacted a code of law known as the Yasa. There isn’t a surviving document of the Yasa Law, and there may have never been one to begin with, but it seems this law was ever-evolving as the Mongol Empire continued to exist. The problem with this law, although a revolutionary legal ‘document’ to the steppe, is that once the Mongols continued to conquer China and the lands of Persia, it would have been very difficult to just sweep away all of the existing laws of these powerful and long lasting societies and replacing them with a set of rules from the steppe. The Mongols relied on their distant relatives like the Khitans to administer places like China and Genghis Khan would rely on the Uyghurs to administer parts of Central Asia and Persia early on until the Ilkhanate decided to hand day-to-day administration over to the Persians in those regions.

Keep in mind that the above information is based on the period between Genghis Khan and the civil war. The way the Yuan administered China is an entirely different beast.

Your question was very broad so I tried to be broad in my answer, if you’d like me to elaborate on some specifics or dive deeper on some of these topics or if your ‘etc’ turns into something else, just let me know.

There's several older threads on this, at least from the Mongol perspective, but I happened to have a recent one open and figured I'd post it for you to read while you wait for more.

What caused the Mongols to invade most of the Old World? by /u/The_Jackmeister.

How did the Mongols rule and maintain the lands they conquered? from a couple of people.

There's also the FAQ which touches on some similar topics, but not quite what your after logistically.

Is this question answered in another thread?