Regarding how mongodb is running whole application as another user (as user mongodb)

[radha krishna rayaprolu]

https://github.com/mongodb/mongo/blob/master/debian/mongodb-org-server.postinst I could see in the above link that mongodb has created a newuser named 'mongodb'.and I got to know that many bigger projects run their project as a custom user (here as a user 'mongodb')..how does actually bigger projects acheive this .what is the command they use to make run the whole application or package as a custom user? In a whole lot reading about this I found that we could use sudo -u <username> <command> or by setting euid or uid by seteuid() function. but I want to know how actually real projects implement it and follow that standard way in my contributions.