Where to check sub msg ready in ROS intra-process communication

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I’m using ROS built-in intra-process communication, with SingleThreadedExecutor. When using DDS communication, ROS check whether the subscription new message arrives from top to bottom sequently in

get_next_executable

wait_for_work

rcl_wait

rmw_wait

But when using intra-process communication, I noticed that the subscription new messages were not be checked as when using DDS communication, and the subscription callbacks were executed as rclcpp:waitable.

So I want to figure out the mechanism of waitable and where subscription new message availability is checked when using intra-process communication.