Don’t make it rocket science logic, it is not. Your first mistake is thinking “ mercenary “ is bad or derogatory. It is not.
The definition is simple. The connotations are broad.
If you are a citizen of one country, and go to fight in hostilities for a foreign country who pays you to use your skills, you are a mercenary. Very easy to understand.
The ammo truck driver, the medic, the aircraft mechanic, the pilot, the computer nerd, the force protection specialist, the SARS guy, etc…
There is a legal definition of mercenary. That definition is very important since it effects the rights a prisoner of war can expect according to international law. Every military contractor I ever worked with before going into a foreign country (war zone or not) was damned well aware of their legal status (or lack thereof) and exactly who to contact if they ran into trouble.