The URL of the Internet Computer is omitted in the spec, but arguably well-known (https://icp0.io).
Coming back to your specific question of accessing public custom sections in the wasm of a deployed canister: you can use the dfx canister metadata command for that, e.g.,
dfx canister --network ic metadata qoctq-giaaa-aaaaa-aaaea-cai 'candid:service'
would display the candid interface of the NNS frontend dapp canister.