Where do iMac custom builds come from at the technopoint?

Question

Online shopping

Hello.
I wanted to buy an iMac, the choice of stores in our city is small, where they can bring equipment, mainly technopoint and citylink. Looking through the list of products, I found that there are a lot of custom assemblies that are incomprehensible, which are not on the official Apple website and in the Re-Store. There are only 3-4 basic models. I called technopoint support, they said that they do not have such information.
Where do such motley assemblies come from, can you trust the purchase of such an aimak, where does the technopoint take them? For example, 7700K, Radeon Pro 580, SSD 512Gb, no HDD.
In addition, it is suspicious that you can order a top-end iMac Pro at almost half the price, although Apple does not officially import it to Russia. Where do they come from?
I really hope for your answer, because I would not want to get into an unpleasant situation and buy the left slag.