What is the specifics of NAT Overload (PAT) for connecting from outside to hosts?

Question

Computer networks

Good afternoon.

There are many manuals that describe how a host connects to a server via PAT. But what about the reverse situation?

How do torrent litches find seeders to transfer files, since our IP is hidden and there are no NAT translations at this moment either?

How do messages come to our PC in ICQ/telegram if we are not writing, but to us?

Or in Teamviewer connection between computers?

Do applications support TCP sessions with servers as long as they are enabled on the computer? Without active broadcasts, it is impossible to send a packet to our computer.

Do I understand correctly that on the Internet, in fact, any connection between end computers always occurs through some server in an open network and there simply is no direct connection, except within a subnet?

And do I understand correctly that the packet goes through PATs more than once and changes ip addresses, since the provider also has a PAT between my router and their router at the AS border, which receives packets from the switch of an apartment building, for example?

spoiler

Да, есть чувство, что я упустил какой-то важный момент для понимания этих процессов и буду рад ответу

Answer 1

How do torrent litches find seeders to transfer files, since our IP is hidden and there are no NAT translations at this moment either?
The answer here is the situation is more complicated, it is connected with the state of the connection and routing, I have not yet finished teaching
https://it.wikireading.ru/12810 (there is such a thing as
upnp and opening ports)
do not we write, but to us?
Answer The
message goes to a server with an open port and is downloaded
from there. Or is there a connection between computers in Teamviewer?
Answer
https://en.wikipedia.org/wiki/UDP_hole_punching
https://en.wikipedia.org/wiki/TCP_hole_punching
A third intermediary node is needed for coordination, and then directly.

Answer 2

Do I understand correctly that on the Internet, in fact, any connection between end computers always occurs through some server in an open network and there simply is no direct connection, except within a subnet?

Correctly.
The topic of the connection behind NAT - it has been reviewed, revised, written, redistributed a hundred thousand million times.
It is IMPOSSIBLE to connect to a server located behind NAT on your initiative (and not a server) . That's just technically impossible. Each subnet surrounding your ISP's network may have its own 192.168.1.1 :) Where would you like to route the connection?
All services for pairing computers of the TeamViewer type work on the same principle - a computer behind NAT initiates a connection to the TeamViewer server and registers there. The other is registered there in the same way. And TeamViewer ensures their connection through their servers
. The same with everything else - there is always and everywhere a control ballserver.

Answer 3

In most cases, a third-party server is used, they are also a kind of proxy server that pairs host connections for nat. There is also the udp-hole pushing technique (only works for udp.