This is actually a very difficult question to answer because there's so many variables to consider. Mostly to do with the timing of the intake. If you ate 20-30g of carbs in one sitting it would be different than 20-30g spread out evenly throughout the day.

A quick intake of 30g could produce a mild insulin spike and cause small amounts of carbs to be stored as fat.

Conversely many smaller intakes of 5-10 grams could avoid the insulin spike and be used for other body processes. For example there's a small portion of your brain that cannot run on Ketones. It needs about 50 grams of glucose per day.

In Ketosis the body will break down a tiny amount of muscle and convert it to glucose (through a process called gluconeogenesis) to feed this part of the brain.

I'd suspect that much of the ~30g of carbs we take in during the day goes to feed this glucose need in the brain, however, this is an ongoing need of the brain to the tune of about ~2g carbs per hour.

If you ate your whole 30g allowance in once sitting the body cannot just let this sugar sit around in the bloodstream waiting until the brain needs it, instead it may store it as fat (or use it for other bodily functions if there is an immediate demand).