In case of Nexus 2 there is the option to get the remote URL for a proxy repo, and the member of a group repo using the /all_repositories and /repo_groups endpoints. In case of Nexus 3 according to the documentaion:
I suggest you update to the latest version, then you can use {{baseUrl}}/v1/repositories endpoint to list available repositories, and then ask for repository details at {{baseUrl}}/v1/repositories/:format/:type/:name. Example outputs are: GET {{baseUrl}}/v1/repositories