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.
[1] https://lafibre.info/images/pon/201012_SFP_Free_FTTH_10G-EPO...
[2] https://lafibre.info/gpon/olt-sfp-possible-revolution/msg433...