I appreciate you for posting here in the Community space, @Mhekang123. Let me help you set up your customer for their specific statements in QuickBooks Online (QBO).

Directing to set up your customer fields on their specific area and the unit number isn't available yet. As a workaround, we can add the customer in the Customer section to edit the Company name, and the Customer display name, add the area and the unit number to identify their specific info, and fill in the necessary details in the field.

For your second concern, we can create SOA with past and present billing statements to show customers summaries to prepare their invoices, payments, credits, and balances. Here's how to do it:

1. Log in to your QBO account.
2. Click Customers & leads, then select Customers.
3. Select the customer you want to make statements for. 
4. Select the New transaction, then select Statement.
5. Review the customer’s email address, then select Save and close.

You can refer to this article for more details about other types of customer statements in QuickBooks: Create and send customer statements in QuickBooks Online.

On your last concern, you can create a recurring invoice to adjust the interval and start date to send it on the exact month you need and check the box to send emails automatically. Once done, the system will execute the command for you.

Here's how: 

1. Go to Settings and then Recurring transactions.
2. From the Action column, select Edit for the transaction you want to modify.
3. Under the option, mark checks the Automatically send emails.
4. Input the Interval and the Start date.
5. Press Save template.

I've included this article about the process: Create recurring transactions in QuickBooks Online. This section includes sub-articles that guide you in managing recurring transactions and troubleshooting.

additionally, you can visit this article to learn how to send invoices over time as you work off an estimate: Set up and send progress invoices in QuickBooks Online.

If you have further questions about managing customers' invoices and creating statements or any QuickBooks-related concerns. Don't hesitate to leave a reply in the comment section. We're always here to help you. Keep safe.