Mathematically, in GPG, what is the relationship between a PGP key and it's subkey

I'm coming from using plain old RSA with OpenSSL for all my asymmetric encryption needs, which I leaed fairly well, but I'm having a bit of a hard time wrapping my head around the OpenPGP protocol. Therefor, this is going to be a few questions.

In my Debian box, using GPG, upon creation of a master key in the current keychain, I noticed the default creation of a subkey. I leaed, after a bit of reading, that GPG manages keys this way automatically; the master key used strictly for signing and the subkey used strictly for encryption. This led me to believe it that they just had different names for a private and public key: The master key (like the private key) was used to sign data that only the subkey (the public key) could decrypt, but the subkey (the public key) could only encrypt data, which could then only be decrypted by the master key (the private key). Am I correct in this assumption, or are they 2 separate key-pairs altogether?

If they are 2 separate key-pairs, what mathematically binds the subkey to the master key?

Is it just GPG that uses this method, where a subkey is created automatically for encryption, or is this mandated by the OpenPGP protocol?

When I upload a key to a keyserver, which key is uploaded, my master key or the subkey? Or both?

When i use the --export function, which PGP key is exported when i specify my UID?

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