Absent any leadership or administrative bureaucracy, how do they make decisions? Is it a question of anyone who wants to, developing a policy and submitting it to the popular vote? How do they envision the institutional structure of a post-capitalist/post-revolutionary state?
so, again as I understand it.
assuming you've got enough members to have this problem, they're divided up somehow - usually by craft I think tho maybe not always - into groups small enough that they can generally make face to face, consensus decisions. when a decision is required that effects more people either there are intermittent larger meetings or representatives empowered only to voice the consensus of their smaller group. & so on & so on to a higher & higher level. the representatives are also I think rotated fairly often to prevent them from becoming like elected offices.
as far as who puts forward policy, yeah I think it's anyone can.
look all this stuff is formal, on paper, in theory, etc. in practice it's always imperfect. there are always problems with a power structure of some kind developing, of certain people becoming leaders. the FAI, for example, functioned as a more militant union-within-the-union & a challenge to CNT bureaucracy to the point where some more moderate CNT members split off to form a non-anarchist Syndicalst Party.
have you ever read
The Dispossessed by Ursula K. LeGuin? that should be added to the general reading list - by far the best theoretical overview of how a large scale anarchist society might function in practice, for better & worse. & she addresses most of the stuff you're bringing up better than I possibly could.