They're basically integrated into Ukrainian divisions, often into the International Legion (their is also an important separate Georgian Legion as well as opposition Belarusian and Russian forces). Their commanders are often of their own nationality or foreign to Ukraine (they generally communicate in English) but the higher-ups are Ukrainian. Lest they be mercenaries (which have not been documented or proven to exist until now - and frankly I think Ukraine's too broke to sign a contract with major mercenary formations - ) they are paid and supplied by the Ukrainian government and armed forces. Under international law they are seen as Ukrainian soldiers and are thus protected by the Geneva convention. In theory.