Not exactly. It's not 24 hours. It's the previous day. For example if your flight is at 3pm on Friday you can have a test done at any time on Thursday. It doesn't have to be done after 3pm on Thursday, you could get tested at 10am. A test Friday morning would also be fine. And it doesn't have to be a PCR test. It can be an antigen test.
Yes, at the airport (Orly, too). They ask that you reserve but walk ups accepted if there is room. They also make it clear that it should be a last -resort since if you test positive, they will immediately want you to isolate. They infer that such isolation means an escort via private transportation to a hotel at YOUR cost!