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I assume signal is udp hole punching to get around NAT.

Signal uses local networking for the account migration functionality: https://support.signal.org/hc/en-us/articles/360007059752-Ba...

You scan a QR code with one device and it transfers the entire account state to the new phone.

What's some good resources on understanding NAT and udp hole punching that explain it in an intuitive manner?

The simplest take on the concept is get a 3rd party with a public address to exchange the current port tuples used to connect to it between the 2 clients so the clients can then use this information to connect directly.

Beyond the basic take on it there really isn't an intuitive single explanation because "simple" things like "NAT traversal" quickly turn into "Full-cone NAT to Port-restricted NAT with UPnP behind CG-NAT" individual corner cases endlessly fighting the need to just go to IPv6.

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