There’s a few ways to handle, but for example:

• Roads: large towns and cities would mostly handle their own road maintenance. Roads connecting towns would probably be joint ventures. Projects would be funded and contracted by the towns and financed by town income tax. Rural areas would be underfunded, but that’s partly intentional - dense population centers are more sustainable.

• Environmental regulations: handled at the level of impact. for example, water quality standards for a river bind everyone who accesses the river. restrictions (e.g. standards for heavy metal levels) would be passed by minority vote - if 40% want a standard, that’s enough. carbon credits would be administered at the Federal or World levels, by a combination of central government and treaties.

• Education: probably pretty devolved, mostly a choice by municipalities in what they offer/teach. there’d likely be standardized tests that most places agree on for transferability (e.g. how the SAT works today.) religious schools could exist in religious communities, or you could have a Montessori program in your secular socialist Kibbutz.

• Slavery: illegal at the Federal/World level. same with indentured servitude and coercive contracts. one of the most important functions of the central government is to protect the civil liberties of individuals.

So the principles are mostly:

• Externalities are handled at the level of their impact.
• More power locally, less power centrally. City governments are more like micro-nations bound by a sort of EU.
• Cities largely have a lot of direct democracy with some representatives. Critically, city governments wield lots of power over the businesses that operate in the city. This is critical to check corporate power.
• Federal government exists as a backstop to safeguard fundamental rights and for truly national concerns.