I've met a new cousin so how does he see my tree and me his? Is there some email attached to my tree? I don't understand
Within your Familysearch account you can enable Relationship Viewing (Account> Settings> Permissions> checkbox for Relationship Viewing). Once your cousin has a FamilySearch account and does the same - you can view your common ancestor and the related descendancy tree (shortest relation path). There is also a BYU Relative Finder application which allows you to explore cousin relation/tree (https://www.relativefinder.org/#/connect)
This is being moved from Indexing to Family Tree.
The Family Tree on FamilySearch is a single, open-edit, collaborative tree -- neither you nor your cousin have your own separate trees here, except for the living people you've entered. (Profiles marked as living are only visible to the person who entered them.) You can give your cousin the profile ID of one of your deceased mutual relatives, and he or she can then easily view and edit the same profiles as you.
As has been stated, you will not be able to view any living persons your cousin has added to Family Tree and, likewise, he cannot see those living persons you have added (to your common, family branch). However, once (after a generation or two) you get back to your deceased ancestors, these can be edited / viewed / merged by you, your cousin - or any other Family Tree user.
There is general encouragement to merge duplicate records in Family Tree, but (for privacy reasons) living persons' records remain hidden from all except the person who created them, so have to remain duplicated until these individuals eventually become deceased.