How to pierce NAT?

Question

Programming

We have 2 applications.
Applications interact with each other.
2 computers (smartphones, etc.) on which applications are running can generally be connected to the Internet in any way.
It is clear that as a reserve there should be a method of interaction through an external server.
But, if specific conditions allow, I would like to get direct interaction, past the server. The server can be used for initial acquaintance of clients.
Strictly speaking, the names of technologies are of interest. What are the keywords for Google
Next - self.
For the universal case - any NAT.
For certain there are standard procedures of a bypass/friendship with NAT.

Answer 1

TCP / UDP hole punching, lots of articles

Answer 2

There is no universal. Above, the pickup option was given, but this is not always and not everywhere.
Therefore, probably the only universal option is the presence of a third party (server). The rest is to your taste:
- all interactions through the server (such as Hamachi, ammy, etc.)
- the server works only as a pimp (torrents)