How to determine what loads the server?

Question

Nginx

Nginx+varnish+mariadb server. Wordpress installed. Bot protection, firewall.
I see that this is mysql, but I don’t understand why it is loaded. Traffic 0. Over time, memory usage grows and reaches 95%.
Help, how can I find out what exactly creates a load on the server, which script or request, which process?
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Answer 1

If CPU load - turn on slow log and study, also SHOW FULL PROCESSLIST;
If there is not enough RAM - look at the output of mysqltuner, obviously the settings are not for 2GB.