The ISP provides a Fiber 10G-EPON ONU which plugs into their all-in-one. [1]
It is my understanding that subscribers with a Freebox Delta only receive this, since the Delta is SFP+ capable. Since I have the older Freebox Revolution the ISP provided me a media converter which accepts the ONU and provides a normal SFP port which can be plugged into a normal device with an SFP port. [2] Then, you just have to configure the ipip tunnel on your device and you're online.
That is not a common scenario, most people in the world get an ONT with an integrated ONU. For example Huawei HG8245Q2. So in your case obviously you can just plug in any router. But a lot of people get it all in one device. And most ISPs don't let you choose what CPE you get.