Hello,

check in specific access> Limit the visibility of resources and define an access mode for the Project leader profile in data access (not dependent on the project) on resources.
I think this will solve the visibility issues

Unfortunately that doesn't solve the problem.

Hello,

If I understand correctly, you want that, when your project leaders assign resources to the activities of a particular project, they see in the list of assignments all the resources saved in your database.

The resources that we see in the assignments on the activities or other plannable elements of your project, are the resources allocated to the project itself.
To see all the resources on the assignments, you will then need to allocate all the resources to the project in question.
There are team, pool, and organization allocation accelerators.
 

I have different project leaders in our company who manage/plan their own projects. But the ressources they have are not only for them but have to be shared amongst the project leaders. So it would be helpful for the project leaders to see wich projects the ressources are planned for. Unfortunately in the view Planning->Gantt views->Ressource planning the only see projects they are assigned to as project leader...

you should try the colored Planning report per resource. It should match your expectations. You can see all the resources and the projects on which they are planned.
A little advice, put colors on your projects so that the projects stand out well on display

But in this report you can't see wich projects and activities the are assigned to

Hi,
Default configuration is that project leaders see resources allocated to their project. This is the case for instance Resource Planning.
This is defined on specific access right "Access on Timesheet for resources" (this parameter is used for other means than just timesheet).
Default value for PL is "His projects", you may change to "All resources"
Other option is to allocated resources to a project the PL manages.