It really is as "simple" as that. While there are many biological processes that try to maintain homeostasis and weight, if you ingest less calories you will eventually lose weight.
Think of it this way ... if you were thrown in prison today and they gave you only essential nutrients to survive, you'd lose a lot of weight. Your body might go into starvation mode first and try to fight against it, but without any calories coming in, you will lose weight 100% of the time.
I decided after years of wanting to lose weight, but doing nothing about it, I would just give up and do 1,000 calorie max / day for 3 months.
I lost 20 pounds to bring my BMI in-range without any other changes, and I'm middle aged.
That's not the healthiest way to go about this, but it does work.
I actually wanted to lose the weight for my 50th birthday, so I'm not some 25-year old with the metabolism of a greyhound.
Just in case anyone is interested: I have a Withings/Nokia scale that syncs to my phone, and then used the Android App Libra to plot a trendline for my intended weight loss and compared that in the app against the trendline it drew through my actual weight loss. I also used the free Samsung Health App to get the calorie values for what I ate, or used the ones on the packet.
I tracked my calories, and stuck to the daily allowance it recommended. I lost nearly all the weight by the agreed timeline (I lost 14.9kg in the end).