On which server does the cron job run by default?

When your architecture includes more than one web(web-xx)/dedicated(ded-xx) servers, the cron jobs run on the lowest number server. For example, if you have 2 servers -

web-998875
web-999876

- the cron jobs will run on the server with the lower number: web-998875.

You can check the server names per environment in the Servers page on the Cloud UI.