How to get variable data from a nested project?

How to get variable data from a nested project?

Hi! I got a general proyect called PROJECT_MAIN.
Inside of it there is other project: PROJECT_NESTED_1.

How can I access variables defined in PROJECT_MAIN from PROJECT_NESTED_1?
And how can I access variables defined in PROJECT_NESTED_1 from MAIN?

Sometin like this (that doesnt work):