So when an apartment pays, is 4 entries in the single transaction? Credit their apartment account, debit the bank account, credit cash and debit receivables? Never thought of making 4 entries for a single transaction.

You would have accounts receivable as a super-account for all your apartments. Its value would be the sum of the sub-accounts. The bank account would be a sub-account of all your cash accounts. So you only need to make two entries, one to the apartment's AR and one to the bank account.

You should do yourself a favor and read a good textbook. The online resources are not great and are mostly geared toward getting you to buy a product. For example:

https://www.worldcat.org/title/accounting/oclc/1127092943&re...