Here the answer from Support:
In regards to how it calculates send time for subscribers which are
newly added and don't have any email send or open history, it won't
send to them straight away. Einstein gathers data for approximately 20
factors, such as the number of emails sent to a contact, the number of
emails a contact opened, and the day of the week the email is opened.
It uses machine-learning to weight each factor according to its level
influence on predicting future engagement behaviour. Then Einstein
analyzes this data and assigns a likelihood score to each of the 168
hours in a week for each contact. Send Time Optimization uses this
information to recommend the best send times for each contact.
I will continue asking about more details because i think the answer is not complete. You can leave comments under this answer for the remaining questions you have. I will transfer them to the case I opened.