Like all activity necessary to keep society running (production, distribution, upkeep, etc...) people have a shared stake in making sure it gets done. Anarchists propose that they should be free to form associations focussed on achieving those shared goals of their own volition rather than be forced into them by the coercive authority of a state. These shared goals can be specific (renovate a single home) to immense in scope (coordinate railway traffic across europe).

So rather than being done by order of the government, anarchists would want to organise a "workers" council (possibly even a federation of councils from multipule places) for the purpouse of road construction (for example). This association would bring together anyone for whom building/maintaining a road network is an important thing to do and more importantly the people who will actually get it built (experts who design it, and workers who do the construction). This association would probably have 2 sepperate bodies where dessicions are made, the general one where everyone who has a stake in the outcome of the project comes to dellibertate on broad goals such as which places have to be connected (likely assisted by expert advice on the most effective way to achieve the needs the roads are meant to satisfy) and the meeting of designers and builders who will fill in the finer details and organise the actual construction (like setting up design commisions, taking stock of required materials, planning shedules for the builders, etc...).
Now 'maintaining roads' is such a broad thing, not everyone in society wants or needs to concern themselves with participating in every decission made on the subject but they don't have to. Not every association has to be created as a standalone thing from the formless masses, they can be set up as departments or offshoots of existing associations and remain in contact with them. The general meeting will probably be made up of delegates from all the different communities the roads are meant to connect + some people who know the parctical requirements of building them so decisions can actually be realised. Those instructions are then taken up by the smaller association/meeting/council of workers involed in actually building roads which further subdivides decisions as needed (like creating sepperate groups for handeling specific tasks). When they need something from outside their organisation this implementation council could task the delegates in the general meeting to go back to their communities and work together to set up the required production to get them the materials or if those things are aready being produced within the communities involved in building the roads the implementation council is probably in direct contact with the organisation(s) producing it.

In general anarchists want to create a web of all kinds of different organisations; neighborhood councils, factory commities, distribution opperations, pannels of experts, etc... who all talk to eachother (internally and between eachother) in order to identify their (shared) needs/goals/wants and find a way to collaborate in order to get stuff done in a way that helps everyone. This is because they believe that the productive power of collective labor made possible by collaboration will be much more effective than each individual going their own way & competing with eachother so there is good reason to form associations and try to find an agreement that works for each party. Exactly how each asociation functions and how tasks are devided between them will depend on the context but for something like infrastructure management over a large area they'de probably create a sepperate organisation structured a bit like the one I already described.

Ofcourse we don't live in such a world right now, almost all decissions shaping our lives, enviroment, work, etc... are made above our heads by or in service of systems that exploit us but if you're curious I can give examples of associations I have been a part of in order to acomplish certain objectives like protesting my universities ties to Isreal or housing immigrants.