Bi-directional calendar Sync with Outlook Client (incl Outlook 365)

I have an appointments website thru which the registered customers can offer appointments to their visitors. When the visitor will create an appointment, the platform sends email confirmation with .ics file to both registered customer & visitor. They currently include this ics file in their calendar to view the appointments. This should be done for every appointment and is tedious, time-consuming, if the customer would forget to do it, they cannot see it in their default calendar. The default calendar that many of my customers use is Outlook Client software installed in their PC. This includes Outlook 365 & older versions. You can exclude the unsupported versions from this list. I would like to have bi-directional sync between My appointments system and the customer's Outlook Client. The following should be possible (but not limited to) 1. I should be able to see the appointments created in my appointments system in the customer's outlook client software, immediately after they are created - This means that I don't have to add the individual ics file anymore 2. When I change the appointment in my appointment system, it should reflect on Outlook Client 3. When I change the appointment in my outlook client, it should reflect on my appointment system 4. When I create an appointment in my outlook client, it should reflect on my appointment system 5. The appointments shall be only single appointments, but can be all-day or specific time with a day or across days. For Ex, 01.02.2021 - all day, 01.02.2021 between 09:00 - 10:00, 01.02.2021 - 03.02.2021 (3 days appointment - all days) You will create a Windows Outlook Plugin (using .Net, preferrably C#) & the necessary API endpoints (using PHP CodeIgnitor 4) in my appointment system to achieve this. Authentication such as API-key configuration should be implemented so that u can uniquely identify the user.