I stated a generalization based on reading widely for over a year. There isn't really one source or even a few, it's from many of varying quality and biases, including the UA MoD, the RF MoD, US gov, independent journalists on the ground, interviews with civilians, interviews with soldiers, military analysts (usually German and Austrian), and reading between the lines when, e.g., von der Leyen makes an oopsie about casualty numbers.