>Does money exist? If not, how are workers compensated for the full value of their labor?
The workers own the products that they make, and if made in join production then I expect some calculation to be made in terms of socially necessary labour time to be used for calculation. Another option is labour vouchers, which are distinct from money because they are non-exchangeable with others and one cannot accumulate more than 365*24 per year (as they are hour based). This puts an end to the function of capital accumulation which is found in money.
>Does a central bank exist?
Probably not. I expect there to be multiple communities each maintaining their own banks of extra produce from which people can either withdraw freely (higher stage of Communism) or with labour vouchers (lower stage). Many would disagree with me on this point, and I'm by far the most well read in terms of distributive aspects.
>externalities
The absence of the state does not preclude collective action or sets of rules. In the same way that if someone appropriated means of production and forced people to work the machines for wage, I would encourage revolution against them, even in an anarchist society.
>trash collection, research, maintaining
Undesirable jobs would probably be allocated on a rota-like system, or some other democratically decided way. Research and maintenance doubtlessly people find motivating in themselves, so I think that would be covered by self-interested parties. I know a few people who would likely be interested to do one or the other. Education would naturally focus around the fulfilment of jobs which are short in supply.
>Who pays the laborer who produces capital?
There is no payment, this is the whole point of Communism and anarchism. Wage labour is off the table.
What kind of capital production are you talking about? If talking about the development of constant capital (materials, machinery, tools etc.) then the allocation of people to make these resources is probably allocated such that each person needs to do the minimal amount work, either in exchange for labour vouchers or a share of the products made with the materials, machinery and tools that were produced. The same goes for the maintenance of that capital, as a machine degenerates by its labour content leaving it into the goods it produces, and it is repaired by supplying it with new labour. This seems difficult to calculate.
>I get vastly different, mostly non-answers to these questions from different leftists.
That's mostly because "leftism" is a massive variety of tendencies, factions, parties, individuals, theorists, ideologies. They (like mine I assume) are non-answers because most people don't don't devote much time to think of this when they have more motivation to study the critique of capitalism, or just do other things. I have to say that most people are simply unmotivated, and I am one of them. People like the original anarchists, Zizek, Badiou, the Marxian economists etc. have much more time to devote.