How can i insert a package im using it for my FPS game
If you have your package already ready, its a simple process,
you go to assets > import package > custom package (or else choose any of the packages listed below.
If you are using a package or using a scene and you want to use this same package in an another project, you can select all you items in your project view, right click and select export package, (note that all related scipts/assets will be exported as well)
Best practice is having all you need in a scene, and export your whole scene as a package.
Then you can import your own exported package in an another project.