How did water get into the footwells in my 2021 Renault Kangoo

Rain water is very good at getting past rubber seals. So pretty much anywhere where you have a piece of glass (windscreen or side windows) sealed against the rest of the car, you have a potential leak point. But water can also get past other types of rubber seals, including the ones that are supposed to seal the door and hatchback/boot openings. Even a small twig that gets caught on these seals can allow water in during a rain-storm.

The other possibility is that the air-conditioning drain hose is blocked or dislodged. This hose is designed to drain the moisture from the air (which separates when the air-con is used) from the air-con unit in the car to somewhere outside the car. If the drain is blocked or missing, wet carpets are another very real possibility.

But the fact that you had overnight rain before noticing the problem suggests a rubber seal is either dirty, torn or missing or has simply gone hard with age and allowing the rain in. Start with the windscreen and work your way back. A good way to find the leak is to sit in the car while somebody sprays it with a garden hose. From inside, you might see the drips of water as they make their way in.