Digital certificates - concepts and implementations

Hello, all right?

I need your help with a digital certificate / digital signature.

First questions are about concepts, for example:

I search on google what it is and does and always appears as people talking about different things, so far what I understood was the following:


I saw that digital certificates need not be made for these certification companies, we can generate the certificate for the Java using keytool, or I can generate the digital certificate without a problem, okay.

This digital certificate generated. Jks q I can say is my "private key"? the file. x509 q I can say is my "public key"?
in my case, I have several users accessing the system, each user will have a public key. x509? or is a public key and use all the same?

1 private to N public?
1 private to 1 public?

Now doubts over implementation, the article demonstrates how to create it. Jks, create. X509, create methods and use them, but does not show how and for what use. X509, which would be the case that the user will only have the public key when the system receives this key, I would like to check if the text encrypted with that key is valid?

And String to encrypt with the public key is the same thing?

These are the questions,

Thanks in advance.