The volunteers are mercenaries of which there are plenty on either side from different countries. Plenty of Americans fighting on Russian side too.
North Korean troops on the other hand are offcial request by Russia and with Kim Jong fat blessings with official ceremony this is due to their defense pact. They where used inside Russia when Ukraine invaded mainland Russia Kursk region which was a failure by Ukraine. Because of which Russia said they will need a buffer zone through out its border sharing Ukraine.
The British Storm Shadow missiles and other stuff needs British specialists. A lot of Nato personal are involved in strategy and decision makings on the battlefield in official capacity. Thats the difference between them and the average mercenary. When ever there is a high value attack on Russian SAM installation or some USV attacks on port, you always find a Nato drone or RC135 flying just outside Russian air defense identification zone. So there is deep Nato involvement in Ukraine. Nato aircrafts are also used for relay for drones and USV long before starshield.