https://www.electricitymap.org/zone/FR
What you'll see there is as you'd expect if you understand how fission power plants work and how economics works for electricity supply. The nuclear plants may not be running at what is notionally 100% nameplate power output but they are not in fact being used as a "buffer option" very much.
That big yellow-green splodge on the "Origin 24 hours" chart? That's nuclear power. Varying, maybe +/- 2GW , but nowhere close to enough to offset France's varying power requirements over the course of a day.
France's (much fewer) gas turbines are much more able to spin up and down quickly to benefit from transient utilization and so they, together with the interconnects possible due to France's relatively central location (the UK to the West, Germany to the East, Spain to the South) allow it to manage well on most days without tinkering with the power efficiency of the fission generators.