CSVBulkUploadBatch.cls

global class CSVBulkUploadBatch implements Database.Batchable<sObject> {
    List<Account> accountList;

    public CSVBulkUploadBatch(List<Account> accounts) {
        accountList = accounts;
        system.debug(accounts);
    }

    global Database.QueryLocator start(Database.BatchableContext BC) {
        // You can use a simple SOQL query here to select all the accounts you want to process.
        // Modify the query based on your needs.
        //return Database.getQueryLocator([SELECT Id, Name, Description FROM Account WHERE Id IN :accountList]);
        return Database.getQueryLocator([SELECT Id, Name, Description FROM Account]);
    }

global void execute(Database.BatchableContext BC, List<Account> scope) {
    List<Account> accountsToUpdate = new List<Account>();
    List<Account> accountsToInsert = new List<Account>();

    // Loop through the CSV data (accountList)
    for (Account csvAccount : accountList) {
        // Search for an existing Account record with a matching name in the scope
        Boolean accountExists = false;
        for (Account acc : scope) {
            if (acc.Name == csvAccount.Name) {
                // Account with the same name exists, update it
                acc.Description = csvAccount.Description;
                // Update other fields as necessary
                accountsToUpdate.add(acc);
                accountExists = true;
                break; // Exit the loop once the matching CSV data is found
            }
        }

        // If no existing Account record is found, insert a new one
        if (!accountExists) {
            accountsToInsert.add(csvAccount);
        }
    }

    // Perform DML operations here to update and insert Account records
    if (!accountsToUpdate.isEmpty()) {
        update accountsToUpdate; // Update existing accounts
    }
    if (!accountsToInsert.isEmpty()) {
        insert accountsToInsert; // Insert new accounts
    }

    // If you need to return results, you can do so here, but for this example, we're not returning anything.
}

    global void finish(Database.BatchableContext BC) {
        // Any post processing if necessary.
        // This is where you might send notifications or log batch job completion.
    }
}