Where is the url for the API?

I signed up and received an e-mail with my API key but couldn’t find a url for the json page itself. What am I missing?

By JSON page you mean the API? in which case you can find the base urls in the docs:

For the Cat API or Dog API it’ll be either:
https://api.thecatapi.com/v1/ or https://api.thedogapi.com/v1/

For the Analytics, Auth, Reports, Location, Finance, etc APIs it is generally the same format