[exim] who starts the delivery process?

Helo
I found in the documentation:
Delivery processes may be started as a
result of a message’s arrival, by a queue runner process,
or by an administrator using the -M option.

The question is the following:
Can you tell me who starts the delivery process
when a message arrives?

Or how does the delivery process
know that a message has arrived?

Thanks in advance for your reply.