Ticket time link with plan

I've imagined that the planned work for activity "A" will be recalculated by Activity time + Ticket time.

No, Real work for activity A will be, but left work will be decremented so that planned work remains the same.

I've imagine that the ticket will be automatically link with the resource affected to activity.

No, it is not automatic.

Management of Tickets is based on this rules :
- Tickets are "not planned events" (bugs, issues, support requests)
- Activities are planned
- what you can plan is global work for a team on bug fixing
Then :
- you plan the work on the activity (to be sure no other activity is planned for resources)
- when resource works on a bug, the activity is updated as if the resource has declared working on the activity (but you get more detail)
- as several resources can be assignedto the "bug fixing" activity, there is no way to automatically define Responsible on Ticket (which is unique)

don't see any advantage to enter the planned work for ticket if there are no link between activity and ticket.

There is a link : the real work on ticket is summarized on Activity (and appears on activity line in Real Work Allocation)

For me the planned time shall be the initial planned + the new ticket estimated time because the initial activity still active and the ticket is an extra works that disturb the initial plan.

If ticket work is not part a of planned activity (like bug fixing activity), then you should not select an activity, and ticket work will appear as "non assigned work".