What actually changes when you lose weight

Two different things happen, and they get confused constantly.

You carry less. A smaller body costs less to run, and that is most of the slowdown people notice. Every pound you lose takes roughly 13 calories a day off what you burn. Lose 20 pounds and you burn about 270 fewer calories a day than when you started, so the same food is now a much smaller deficit. Nothing has gone wrong. The arithmetic just moved.

Your body also adapts a little on top of that. This is the part people mean by metabolic adaptation. It is real, but it is the smaller of the two, it tracks how big a deficit you are currently in more than how much weight you have lost, and it eases off when you stop dieting.

The practical difference: the first is permanent and predictable, and your projection already accounts for it. The second is temporary and much smaller than the internet suggests.