How to find out which key to use for a gpg interlocutor?

Question

Information Security

I know the math basics but am a bit confused about the gpg workflow. A couple of questions:
1. I downloaded the public gpg key of the interlocutor from the key server, I import it to myself. How can I / anyone else know which public key to choose if there is a public master key pub , a pair of public subkeys sub and all have the ability to decrypt (E) Using: E ​​? How does the interlocutor on the other side know which key I used? If I chose the pub master key, and on his laptop he has access only to the subkeys (in all articles, the user is recommended to alienate the master key from the working computer)?
2. If I sign messages with a subkey ( sub Using: S), then how do I place the public key certificate on the internet for verification? I did not find the export of the subkey in gpg. Other people have to check the signature with the subkey (how do they know what subkey is signed with?), then see that the subkey is signed by the pub master key and look at its certificate to find out the USER ID , right? What is the general workflow in case of signature verification made by a subkey?

Answer 1

Understood:
1. on the receiving side: --try-all-secrets
2. the public gpg-key on the server contains all public subkeys. The signature contains a subpacket with a key ID. By key id from the signature subpackage on the server, a pub key is searched, and already it contains all open subkeys, including those with the desired subkey id