In my experience, sewage is billed based on water usage, which makes sense to me.

If you use some water, presumably it has to go somewhere.

What if you're just watering your plants?

Here your max sewer bill is set based on the average water use of 3 winter months. If you go over that in the summer, we assume it’s mostly landing in the yard and not the sewer.

You usually can get a credit for la large water usage that doesn't go in the drain. Most commonly done for filling swimming pools.

Our sewage bill is determined by the lowest two months of water usage from Dec-Feb plus a base cost.

They’re separate in a lot of places

Not directly but the reason why you pay such a high water usage rate is because it's indirectly metering sewage usage. That's why you can arrange for bulk water delivery via fire hydrants for a cheaper usage rate for e.g. filling up a pool.