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By default, wp-cron runs every time an HTTP request is made to a WordPress site. For this reason, really popular websites have found it necessary to disable the default wp-cron functionality and replace it with a real cron job.
There are several articles available on how to do this. Using the phrase “Replace wp-cron with real cron” resulted in several hundred examples. This one is fairly generic, but you can probably find one that matches your current host almost exactly.
ADDED FOR CLARITY: WP-CRON is executed as an asynchronous process. Therefore, you will not be able to “hook into” it in the same way as other filters or actions. It serves no purpose to understand “when it fires”, as it could already be running when a user makes any given HTTP request.
If you need to “hook into it”, you should probably view one or more articles on “scheduling a wordpress cron“. In particular, I found this WP Beginner article to be very clear and helpful.
Good luck!