Difference between "Signing identity" & "certificate"?

Question

I was studying how to build iOS apps, but I found two terms very confusing:

1. Signing Identity
2. Certificates

I have gone through this official Apple document. It says Xcode uses "signing identity" to sign the app & Xcode also generates the signing identity. So what exactly is signing identity?

1. Is signing identity synonymous with certificate?
2. Is signing identity username & password credentials?
3. Where are signing identities located? In the keychain access?

Answer 1

1. Yes, more or less. The identity is a private-public key pair in the keychain, and comes from you. The certificate is a way of tying them together for a specific purpose, and comes from Apple. The certificate is Apple's way of saying, "I have seen this public key and I affirm that it belongs to an authorized developer".

2. No, irrelevant. Your username and password have to do with your Apple ID, which is what you need in order to talk to Apple over the Internet.

3. Yes, as I said in (1), the identity/certificate lives in the keychain, and Xcode can see it and so knows that this computer is authorized by Apple for a specific purpose.