Before any start of coding I have some problem to understand what is the main purpose of the Google Calendar API.
As you know there is some Auth process before creation and enabling API.
That means that I, as owner or developer want to use Google Calendar API so I get client/secret/keys strings and it is OK.
I can create an app where I can “promote” my Google Account Calendar being public and then I can show all events from that calendar (dentist booking etc).
Also I am able to use Calendar in another way. For example: Within my App I can create one page where users can auth to their google accounts and see their events
are already created.
But, What if my logged users don't have a Google Accounts.
Google Calendar is strongly connected to already created google accounts? Is it possible to use Google Calendar strictly as an REST API?
I know that this may be a stupid questions but this is something that most of Google Calendar API beginners have problem with.
There are technically two ways of accessing a calendar on Google Calendar.
Your first option is where you are using Oauth2 to authenticate your users. They give you access to their google calendar and you can then insert events directly into their calendar. You can also see the events that they currently have. This as you said wont work if the user in question does not have a google account.
Your second option is to use something called a service account. Think of a service account as a dummy user. It has its own Google calendar account minus the web view. You could potentially us that to store events in a global calendar application calendar. Then when you want to add a user to an event you invite them you can set notification no they should receive an email and they will them be able to add the event to their own personal calendar. For you this may work out better because it does not require you to have access to the Users google calendar the only draw back will be there is no way for you to see if said user has any events going on at that time since you don't have access to their account to check.
I have given you a couple of links to some tutorials that I have write a few years ago that explains the difference between oauth2 and service accounts.
Google Calendars are tied to users, which means Google users. First of all, to access the API you need a GoogleAPIs developer key. This requires a Google account. Then you need Google accounts to use or test with the API.
The Google Calendar is tied to a user account as described at https://developers.google.com/google-apps/calendar/concepts/. One thing to notice is that "event" is an atomic unit in the API and a calendar is a set of events. In other words, a calendar in the Calendar API isn't a timespan like we think of "this year's calendar", it's a set of event objects. Within the app you describe, if the users don't have google accounts then they don't have associated calendars. You would have to tie these users to some kind of public or shared calendar. It's unclear if using the Calendar API solely as a REST API as you describe (without actual or "verified" user accounts) is in accordance with the Terms & Conditions. That aside, in theory it may be possible to use a service like that as a REST API to suit your needs. Maybe you can try inverting the problem so an event becomes the user with a primary calendar. Now the location of the event can be treated as the API-event. Other (normal Google) users can "attend" the location, at the given time, created by this event (=user). You could also apply the same approach to invert the problem by location. Location becomes the user, event becomes the API-event, and attendees are normal users. The latter approach is used commonly in businesses to book resources like rooms, equipment, etc.