Zabbix stores all it's data in a PostgreSQL or MySQL database. However.... there are 2 ways that Zabbix Agents work. Either in passive mode, or in active mode.

Passive Agent = "poller" process on the Zabbix server sends a request to the agent asking for values for the items it's monitoring (based on template applied to host). Depending on the number of hosts you're monitoring and how many poller processes are configured to start with the Zabbix server, you may run into a situation where requests are queued because the poller process is too busy. Increasing the number of poller processes will fix this, but it also adds additional load to your DB as each poller process will connect to your DB to write data, and each poller process will consume a certain amount of memory. Too many and you'll run out of RAM, or bog down your DB.

Active Agent = "trapper" process on the Zabbix server listens for item values from being sent to it from the agents. Agents will query the Zabbix server to see what templates are applied to it's host, and will figure out what items it's supposed to monitor. The agent will actively query the items without the Zabbix server requesting it, and will send the item values to the server as scheduled. This puts a lot less load on the Zabbix server.

Item values are not read from the DB to activate the trigger. When a value is received that matches the trigger's expression, then the trigger is activated. Live values are used to activate triggers and trigger actions (alerts).