I was curious about how the zabbix-agent (active) works and how often it sends data etc.

It seems there are some configuration settings but one does not get a real feel about what is actually happening. More information at:

https://www.zabbix.com/forum/zabbix-help/458380-how-often-active-check-item-values-are-sent-to-the-server

It would be interesting to be able to monitor a zabbix-agent to determine how much data it is transferring in active and passive mode, how often, how many connection errors occur, how long each connection takes, how many servers it is communicating with, etc. There are probably many interesting metrics which can be collected and used to optimize the system.