Is it possible to create my own custom events in ActivityTimeline?

Yes, you can create your own custom events in ActivityTimeline which will be based on the functionality on already existing events. You need to go to ActivityTimeline Configuration→Events→'Create New Timeline Event Type'

Why ‘Bookings’ are showing as worklogs?

ActivityTimeline allows to display booking events (and any events based on ‘Booking’ event type) to work as Jira worklogs, when they are scheduled in the past.
The reason behind this feature is because ‘Bookings’ events are sometimes used to plan activity that doesn’t have Jira tasks behind it, so in order to be able to properly track this time we automatically count booking time as worked time.
You can change this in the ActivityTimeline Configuration→Timesheet Config→Treat ‘Bookings’ as worklogs:

You can learn more about ActivityTimeline custom events via the following link:

Custom Events