If you have a 21 hours layover in ORY, the best way would be to allow you (almost) a full day to visit Paris. It's very easy to reach Paris downtown from / to ORY airport. You can find all public transportation on the Paris Aéroport site, with their respective price and time.
If, for any reason, you can't or don't want to enter in France and want to stay inside the airport, here's what you must know:
The airport is easy to navigate.
The facilities are fine, and overall the airport staff are generally tolerant of overnight travellers trying to sleep.
Most of the seats have armrests, so many travellers prefer to stretch out on the floor.
Some travellers report being able to sleep on the benches and couches in some cafes.
The airport can be cold, so bring layers – especially if you’re sleeping on the floor.
Plan on bright lights, so bring a sleep mask/eyeshades.
Source: Paris Orly Airport Guide (Sleeping in Airports)
However, please note there's no luggage facilities, so you must watch your luggage until check-in. French airports are safe but vigilance is still required.
There's also 3 hotels you can access with Orly shuttle, but they're not inside the airport itself:
Also, there's no noise issue (at least no more than in any other airport), nor ventilation issue.