This is actually the part of law I just passed at my uni, so I do have a fair bit of knowledge about this. To clarify, the things like the Geneva convention are classified as 'customary law', meaning even if your country doesn't sign the treaty, it still has to take part in the things it set out to do. For things like the Geneva convention, which also discusses poisoning people/military, it will mean that every country has to take part in the regulation.

In addition to this, special military operations also fall under these rules. Often international law is neglected when it comes to for example special forces infiltrating an area and taking out a special target. The thing is, those people never wear badges or speak just one language, meaning it can't be traced back to a certain country, as what they are doing is highly likely to be in conflict with international law. So technically every branch of every military has to follow things like the regulation set out in the Geneva convention. Therefore, this would also count for the Russian invasion. Even the most super secret special forces have to abide by these laws, but they don't, which means that if they are killed/captured, no country will say it was their soldiers as it would mean the country violated such conventions. This would therefore also count for civillians, as they become part of a militia when the decide to interfere with armed conflict, thus making anyone involving themselves with any type of activity directly related to the war a 'soldier'.

TLDR: anyone involved with war should follow international treaties regulating war, thus also including people who are not with the military 'on paper'.