Until you have a construction company in your country you are using foreign resources and labor and must pay for them.

Gameplay wise, there is a button that looks like a crane at the bottom right and one that looks like money, when constructing a building. The crane is for planing construction so your construction companies start constructing them.

Inside the construction company you must select where they get the resources like steel and concrete, usually you want some nearby warehouses, if you make the resources yourself make sure to fill the warehouse with them, otherwise buy them from abroad.

People, if they live close to the construction site will go to work there, if its far away you need to buy a bus in the construction site to move the people.

Bear in mind the game tells you how much working days it is needed to build any building and how many resources you need, its only if you are buying the building when money matters and the game gives you an amount based on how much the labour and resources cost.

When you buy buildings you are buying the resources, having them shipped in, and having foreign contract workers come in with that to actually build it for you using their equipment. It's just it abstracts this away for the sake of gameplay. It's like when buildings are purchasing and importing the goods they need, it's abstracting the actual delivery of them from outside.

It may be a game setting to enable a more complicated build option (so if you're still in the tutorial campaign this may be disabled, I'm not sure), but if you look down at the right corner where you have "finance with rubles" and "finance with dollars" boxes, there should be a third one to the right of them that uses local resources and labor. That doesn't cost money (directly, but if you're filling warehouses of material with the auto-import purchase option that will cost money) but requires construction offices staffed with transport and heavy construction vehicles, as well as busses to take workers to the site, and you need to be sourcing all of those things from somewhere.

someone should write a book about this

Idk how the USSR or it's republics did accounting, but even under socialist conditions it would be conceivably helpful to account for building construction on a per unit basis because it lets city planners evaluate the relative value of labor and resources being put into a building.

For example if you have two sites where you can build the same apartment block but one is flat ground and another is hilly or marshy ground, then it makes sense to calculate the amount of labor, additional resources, and specialized equipment you would need to level the slope or build extra foundations on the swamp.

Lots of companies and organizations also carry out internal accounting where they invisibly pretend to pay themselves and their own divisions for work because it allows management to see where money is being used and how much.

They don't? Like there's cosmonaut mode which I've done, that goes all in on the idea of not importing anything from other blocs except for the starting stuff.

Money is only used for a few things

• Buying resources from the two blocs
• Buying labor from the two blocs
• Buying vehicles from the two blocs
• Inviting workers from the two blocs
• Transport costs when you don't want to pick up whatever you bought at the border

If you have your construction office set up properly then buildings cost literally nothing

You can totally make things without paying for anything other than a minimal starting industry. Look up tutorials for cosmonaut mode if you want to go hermit kingdom. (or if you're really insane, sputnik mode where you install mods turning the place into a horse drawn 1917 economy)