I always found calorie counting annoying and never really trusted it because our metabolisms adjust.

So, I was thinking it would be simpler to just think about mass in vs mass out.

We don't extract enough energy from our food to meaningfully reduce the mass passing through us, which should mean weight loss is just a matter of input < output.

The takeaway is eat/drink less and sweat more.

One of the key mechanisms is the endogenous water that our bodies produce by burning fat. If you allow yourself to be less hydrated, your body will burn fat (and other stuff) to produce the water you need. This is why the amount of sweat and waste leaving your body won't just decrease by the same amount that you reduced food and water intake.