How a software is assigned a designated port by the community?

This is general question on unix software development. I actually have seen some of the earlier questions (mentioned below) which gave rise to my question:

Suppose if you are building a service which you later want to package and publish for the linux distro and it involves networking, if you are making everything my scratch ( like not using ssh or http etc ), then you will need some default ports for communication. So how does the community ( or whoever is responsible ) decides which port to be used and make it reserved?
How the whole stuff for deciding a new reserve port works out?

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Author: Aaryan BHAGAT