I want to speculate and discuss the game's upcoming features, they release dev diaries every Thursday.

https://forum.paradoxplaza.com/forum/developer-diary/victoria-3-dev-diary-3-buildings.1478868/

Last week they talked about buildings. Based on the screenies, I'm making this guess on their design. Pop groups accumulate "investments" (aka capital) through the dividends made on profitable private farms, mines, and factories. My guess is that the player will have access to those investment pools in order to fund more industry within their borders and internationally. Workers make products at a factory. Those are sold and workers and investors make a wage that they use to buy things. If there's a profit because income exceeded expected wages, then that money goes into the factories savings. Once savings reach a certain point, investors are given the rest, turning that profit into capital. The player controls all capital, state and private, owned by people in their country. What this may also mean, is that if a planned economy elimimates landlords and capitalists, it sends any dividends straight to your treasury. I'm sure this will have an efficiency penalty, but whatever, the soviets werent the best investors in the world. This might also mean you could mod in market socialist economies that divide wages equally between the employees, so the player has to rely on just taxes to accumulate capital.

Either way, I'm really excited to see what else they're changing about the game