[SalesForce] how to split string based on particular digit in apex

You can't start with a UTF-8 string in Apex; by then, the damage has already been done by UTF-8 string conversion. Instead, use FileReader readAsDataURL to get a base-64 encoded version of the file, then send that directly to Apex. Your final Apex code should simply be:

Blob fileBody = EncodingUtil.base64Decode(base64String);

As comments suggests there might be some better solutions to get or store InActive Users information rather than using Custom Metadata Type records for it.

But to your question - Sounds like you are trying to get a field value (Inactive_Email__c) from a Custom Metadata Type records stored on InactiveUsers__mdt and then compare it with a different string value received as Input or from a different Query.

Below Example can show a way to achieve this...

InactiveUsers__mdt[] InActiveEmailRecords = [Select MasterLabel, 
Inactive_Email__c from InactiveUsers__mdt];
  Set<String> InActiveEmails = new Set<String>();
  for(InactiveUsers__mdt record:InActiveEmailRecords) {
      InActiveEmails.add(record.Inactive_Email__c);
  }
  //Example Query for User based on the InActive Email addresses in mdt
  User[] InActiveUsers = [Select Id,Name,Email From User WHERE IsActive=false AND Email IN:InActiveEmails ];

 // OR If you want to match with a certain string
 for(String email:InActiveEmails) {
      if(email == "YourString"){
          // Your logic goes here...
      }
  }