Economics is merely the study of how scarce goods are distributed. There are many ways to do that. Capitalists argue that their system is the most efficient, but even taking that claim at face value, efficiency alone isn't the only measure. It also says little about how control of those resources consolidates power.

That depends on what your goals, methodology, and material conditions are.

If you have static and predictable demand for something, like, everybody needs enough food to eat and a roof over their heads, then the state can effectively plan for and deliver on that. When you have something that just needs to be done for its own sake, then introducing a profit motive isn't necessary or beneficial, and can lead to enshittification and cutting corners. There are also "natural monopolies," like for example cable where it doesn't really make sense to build another set of cable lines so the consumer is going to be left with one choice anyway, and it would be better for that to be run by the state where there's at least some democratic input on how things are done. There's also what's called "externalities" where for example building a train line makes an area more accessible and brings value to the surrounding area, beyond what can be captured by the line itself - and conversely, there are negative externalities, such as a factory putting out smog or dumping chemicals, that will lower the value of the surrounding area but the factory won't bear the brunt of the costs. There are also natural resources where the supply is inelastic so taxing them doesn't make the economy less efficient, as the Nordic countries do with their oil.

There are situations where private industry can make sense. Like, say people want plushies made of different anime characters. There's no real reason for the state to run that. There's a low barrier to entry, and different companies can use different methods to predict which character will be in higher demand and whether people will want higher quality or lower price. Or with cars, they can design different models balancing all sorts of factors from safety to performance to aesthetics.

There's also many cases where a state doesn't have the freedom to implement whatever system they want. Some states are rich in natural resources, but they are owned by foreign corporations that have owned them since colonial times. If they try to nationalize them, or even tax them, it would be beneficial to the economy but it could lead to sanctions, coups, or invasions. But generally the people in the country don't benefit from those situations and the relationship is parasitic.