Who is leading the project? What does joint project management mean?

For the extension of Metro 1, Île-de-France Mobilités and RATP are jointly the project manager.
Île-de-France Mobilités is responsible for ensuring the feasibility and timeliness of the operation and for determining its location, programme and provisional financial envelope. At this stage of the project, Île-de-France Mobilités has a "prevalence" that makes it the reference project owner for the studies. Île-de-France Mobilités also led the organisation of the consultation.
The RATP, for its part, chooses the process by which the infrastructure and equipment are built or acquired, ensures or has them carried out as project manager and concludes contracts for the studies and execution of the works.