Depending on the material you have piked for the inner pipe, the factory produces the fittings you need for you system. The fitting has the respective inner material (in the jointing technology speciafied) and outer material PE100 with prefabricated welding ends for electrofusion.
In case of a solvent cementing for the inner pipe, please consider the type of cement. If you are using the DYTEX solvent cementing for the system, you should also order the fittings, that have been DYTEX cemented. It is important for certain range of acids.