Integration of WordPress plugin with external database

The essence of the task is as follows. There is a website on wordpress and it has a plugin for making an appointment with a doctor (booker). Thus, it is possible to choose a doctor, date, time and book an appointment. There is also a separate server where all orders, data on doctors, customers, schedules and other information are stored. It is necessary to synchronize information between the server database and the database of the wordpress site. In particular, if the site receives a record of the reservation, then the data should also run to the server. Also in the other direction. If you manually added a record on the server, then the data about this record should be used in the wordpress database. That is, there should always be up-to-date information about free and busy time in the schedule. You will need to understand how the plugin puts data in the wordpress database, in which tables. Database on Firebird server 2.1.7.18553. According to the requirements, probably knowledge of php, wordpress, Firebird and node.js (I think that this is good to do on node but perhaps you will offer another option)