Email-to-Case in salesforce:

https://focusonforce.com/configuration/salesforce-email-to-case-example/

How to get the Case Number using ThreadID from Apex Class?

Using ThreadID, we can get the relevant Case Number using “getCaseIdFromEmailThreadId” method. see the below example for your reference:

String emailThreadId = ’00DD0CG3j._500D0ZaQLq’;
// Call Apex method to retrieve case ID from email thread ID
ID caseId = Cases.getCaseIdFromEmailThreadId(emailThreadId);
system.debug(‘caseId:::’ + caseId);

Same way we can prepopulate the Case ThreadID using formula fields,

Here ThreadID is nothing but its a combination of Your Salesforce ORG ID and Case ID.