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Go check out STUN and ICE.

The best article I've found about NAT traversal is this article from Tailscale: https://tailscale.com/blog/how-nat-traversal-works

Not bad, though you don't even need STUN or ICE;

That uses WebRTC, which uses the same NAT traversal tricks.

is there a filesize limit for this?

You aren't guaranteed to be able to do that but in practice most times you can.