The concept of a P2P, which I discussed earlier before crypto came up, goes like this:
1) I need to transfer, for example, $150 from Russia to USA.
2) You need to transfer $350 as mentioned about from USA to Russia.
Now, if we can trust each other, you see how we could help each other accomplish our goal by just making local transfers between each other rather than any international wires!
However, it's not a complete solution, because there is an imbalance in our amounts, and you'd need somebody else also to get your remaining $200 exchanged.
Of course, dealing with strangers like this who have no reason to trust each other, save the common need for repeat business, usually there needs to be some kind of broker, middleman, security deposit, etc. to provide a means for recourse or chargeback in case either of the parties defaults.
What I've just described is what a P2P exchange is supposed to do: match the parties, facilitate the trade, and provide some means of security. It's all based on an concept called, "Bills of Exchange" which pre-dated modern era bank checks usage.