It "converts" from and to Ethernet frames, of course. There is not actual L2 difference between PON network and normal fiber ethernet network. In simplified description, it just replace active switch with passive optical splitter.
Yes. Like fiber ethernet, differnet speed need different ONT. But noteworthy, there are some different standards for the same speed level, they are incompatible. For example, EPON and GPON, they are both for 1GbE but incompatible.
The green PCB on left-bottom corner breaks the consistence of design...